Circles: Just Like Real Social Circles
Quickly, think of one of your real life social circles. For most people, they will think of a group of friends, coworkers, or family that is tight knit and perhaps share some commonality. In many circumstances, these people will show up in multiple circles. G+ models this perfectly. You can easily take a person and put them in as many circles as you'd like.
Again much like real life, that person doesn't know that you consider them part of a particular circle unless they know implicitly or you tell them. That person may be on your "frenemies" list. Perhaps you only consider them to be an acquaintance (we'll get back to this) but you don't want them to get the wrong idea that you're keeping them at arms length.
You may never use this feature to its full potential, but one of the aspects of social software is that it allows you to organize and catalog your life in new and novel ways. The implementation on G+ is both easy and visual. You may learn something about how you think of your friends by attempting to put them into circles.
Sharing Controls Allow More Frank Conversations
When you share something on G+ a key feature is that you can easily limit or expand the scope of sharing. I know some people consider this to be a confusing extra step but it is necessary to model these interactions. It allows people to conveniently have separate social circles that need not often interact.
Think about it, is your mom or boss on Facebook? This answer is increasingly "yes." With Facebook's privacy settings it is complicated to avoid sharing sensitive information with these people. It's likely none of your boss's business that you were out partying all weekend, but it is so easy to inadvertently tell her just that. In order to avoid this you must either not befriend these people on Facebook (smart, but sometimes awkward) or go through a fairly unintuitive procedure to modify who can see a particular post. It's not impossible, in fact I have custom security settings that keep several people who are officially "friends" from seeing the content on my wall, but it is nowhere near as intuitive nor as central as it is on G+.
As an aside, I think that Google is placing their bets that by enabling you to have more control over who sees what info you will in turn feel more comfortable sharing things. If that becomes true then people who share relatively little now could find new life in a product like this. Also, I should note, the addition control is not absolute. Just like in the real world, if you say something to anyone then that person has the ability to share that information. Digital communications are easier to copy and verify, so it's not like this would give you carte blanche to trash talk your employer or openly cheat on your spouse.
Dealing With Acquaintances and Beyond
The way Circles work will allow for far less awkward interactions with people you don't know or don't know well. If a random person adds you to a Circle, you can simply ignore it and they will only see public posts. Any posts they share with you will go to your Incoming page. You simply don't have to see those people, and it requires no action on your part. Sure, you can block them if you really want. A better strategy would be to simply treat public posts as you would any other public speech and not say anything too personal or socially unacceptable and you don't have to do anything about them.
Say that guy you met at the party last weekend adds you, and you might share some stuff with him based on what you know about him but you don't want him to know too much about your personal life, then file him under Acquaintances. When you share personal info don't share it with Acquaintances. Or create another group that's even less intimate. Chances are that most sharing of this sort barely has a real world model because many acquaintances don't have frequent interactions after the fact. So even if you never share anything with these people you shouldn't feel bad about.
A Conversation With a Circle
In the real world it is unlikely you will have a chance to talk to people from all of your social circles at once more than a few times in your life. The one time this is likely to happen, at your wedding, is something that many people only have happen once, and others only a few times. Good luck getting your coworkers to buy you a fourth wedding gift.
Instead, you probably have conversations with a one circle of friends at a time. It's likely that you tell these groups many of the same things, but you probably choose not to tell certain groups certain things, and other times you probably change things slightly to match the group. Each group likely reacts differently, even if similarly, to the same conversation. Sometimes, you would tell two people the same thing, but not when they are in the same room. That's how social dynamics work. It's a dynamic that Facebook breaks and G+ models somewhat correctly.
For some people, Facebook has changed this social dynamic forever. Any public announcement will be just that: public for all and for all to comment on. They probably value the varied interactions of their different circles of friends meshing together. Fortunately for these people, G+ offers the "My Circles" and "My Extended Circles" sharing settings, not to mention "Public".
For everyone else, the genie can be put back in the bottle. If you have a conversation with one group of friends no one else need know. You can have the same conversation multiple times shared with multiple groups and avoid any interaction.
Why would you want to do this? Well, maybe you want to give your close friends a low down on your trip to Cancun, but you want to share photos with your family. You don't want your rowdy friends commenting where Granny can read. Or, maybe you know people from Ohio and Michigan and you want to discuss the fine mess that OSU's football program got itself into but you'd rather it not become a huge flame war.
A Conversation From a Circle
Here's another key difference. Right now G+ does not have a "wall" that anyone can write on. Some people think this is terrible, others love it. I like it because it gives me control over who sees what my friends say to me. However, the real benefit of this is that it models how interactions from a circle of friends to you work in the real world.
If you're hanging out with a circle of friends one of them might say something to you that everyone in the circle can hear. This could strike up a conversation within the circle, and maybe it's a story you would recount later to others but people outside the circle would not likely be involved.
How you model this in G+ is to make a post directed at your circle and tag the person you are speaking to. This will allow your mutual friends to comment on this post. If your friend wants to share it more broadly he can do so by clicking Share and selecting more of his circles. By sharing it with your mutual circle of friends you can have the same sort of intimate, candid conversations you would have in the real world. If it's something you want more people to talk about you retell it by sharing, the same as you would need to otherwise.
This again empowers you to control who sees what information. If you think about Facebook's wall, the idea of allowing someone to write on your wall is like asking for someone to write graffiti on your house or draw a penis on your forehead. Sure, it's also like having them sign a cast, but even then they normally ask permission. Think of resharing as your wall plus asking permission.
Public speaking is something that Twitter does pretty well. Conversations on Twitter are so disjointed that it is more a broadcast platform than anything else. Of course, conversational discourse is kneecapped on Twitter due to the size limitation. Facebook makes most of the things you say into a semi-public event that is invite only. Unless your profile is open to the public only your friends will see it, but then those people not in your friends list can't interact with it. G+ is modeled a little bit after both services, allowing you to have both private and public conversations. However, G+'s public conversations are far superior to Twitter and more shareable than Facebook.
Anything you post that is aimed at the Public should be considered to be something of a seminar. It's like gathering all of your friends, acquaintances, fans, etc. into a big room and offering for anyone to comment. You can assume that this will be fairly public, as it is tied to Google after all, but the people who will immediately know about it are the ones who have you in their circles. Thus, you practically have an attendance roster right on your Circles page. Unless you disable comments then you can allow public interaction on these items, basically anyone with a G+ account can comment.
Getting Along with G+, Acceptance & Adoption
When we deal with a new service like this one we must be careful. Some people will proclaim it the next big thing, others will call it DOA, and still others will begrudgingly drag themselves along for the ride. We'll recall Google Wave (over and over again) and Orkut. We'll think of MySpace, which is funny because it was a huge success that just didn't have staying power. Maybe we'll think of all the other projects Google has done that no one would give a chance to yet have proven to be popular over time, like GMail, Google Maps, and Android.
Chances are that people like me will be more lenient on the service. I don't mean because of the reasons laid out above, but rather that I tend to love Google interfaces. Even their quirks often agree with me. I try to cheque my fanboyism and be objective. Certainly, as someone who does interface design for a living I can be critical of their choices. Still, it works for me for the most part.
It's also important to remember that this service was launched early in the development stages. It is clear that they intend to follow their pattern of rapid iterations and live testing. Google is capable of developing slick interfaces that work well, but often their first generation is somewhat clunky and pointedly favors geek culture with features like keyboard shortcuts. If you're not so much of a geek (or sometimes if you're just that much of a geek) then you won't appreciate this as much as people like me.
I'm sure there are more ways that this service both mimics and deviates from real life social interactions. After all, it is a piece of software and it does do things that are impossible or difficult to physically accomplish, like bringing together people from geographically divergent places. However, I'm not exactly a social scientist nor will I proclaim myself to be a social media expert. This is all I've thought of up to now and it was inspired by several conversations with various friends. This may not be the last I write on the subject, I only hope that the next thing I write isn't a post-mortem.