Nicole Ellison has a good post on the uses of Facebook as identity affordance over time, and puts very nicely the sentiment that,
this expanded social network of people from one’s history as a supportive presence that enables individuals to stretch, knowing that they have links to their past should they need them.
Ongoing research I’m doing with Terrell Russell examines user perceptions of time and the life-cycle of information, and this adds another useful perspective. But there’s yet another aspect to keep in mind: the way in which our networks don’t limit who we are, but keep us honest on who we hope to be. I haven’t seen research quantifying this, and I’m not quite sure how you’d do it at any rate. But it’s my big hunch that one of the best aspects of explicating social networks online, and making behaviors public and observable (even if we’re modulating privacy settings) is that we are aware we’ll be held accountable for who we think we should be by those closest to us. It doesn’t even have to be explicit, just the knowledge in the back of our minds that we’re being watched on some level by those we we have chosen to support the norms of our created communities is, I think, an important function.
Jacob Kramer-Duffield thinks 
Hi Jacob. I did a literature review recently on issues related to sense of community, community attachment, etc. (in the physical not the virtual world).
Particularly in the criminology/sociology literature is the idea of social control, that having enough people who know you or who are at least keeping an eye out on you or for you can be enough to curb crime. One of the studies I read said neighbors really only needed to see each other once a year (such as at a neighborhood cookout) for there to be a significant drop in burglaries and robberies.
I wonder if, by creating a personal community of long-time friends, we’re creating a form of social control for ourselves. That can be bad as well as good, but it does kind of externalize the ideals we’d like to think we believe in. (But then when you leave a community…when I stopped attending Southern Baptist churches, I lost touch with many people I’d grown up with.)