I'm a big fan of on-line communities. The ability to form a group of like-minded people who may never have met and who may live in different countries is a wonderful tool. I've joined loads of forums and social sites over the last few years. Most forums are fairly limited in their social functions. You may be able to mark certain people as friends and send them private messages, but the overall population will be limited to those interested in the topics discussed there.
I think that the first proper social site I joined was Multiply. I liked their focus on sharing stuff with friends and family rather than talking to the whole world. The way you can restrict access to any item is the best I've seen, but it hasn't taken off in a big way. I got a few people on there and it has proved useful. Much later I gave in to Facebook as there were lots of people I know there. There are just so many things I don't like about it, but I can see why those are part of why people find it attractive. It's a real walled garden that protects you from the wider internet. They do set very low thresholds for forming new communities (groups), but those I have joined do not seem to be used much. People join and then do nothing to contribute. Facebook Pages can act like RSS feeds to keep you updated on your favourite band or other organisation. I just prefer more open technologies that don't tie you to a single site.
I've also joined other networks like Twitter, identi.ca (the Free Twitter), Friendfeed and now Google's Buzz. They all offer the ability to communicate and have mostly been useful to me. I've been suffering from duplication due to people posting to multiple services to reach the largest audience and so have been cutting back on my connections. I've dropped those on Facebook who just posted their Twitter updates and those on Twitter who I had followed at some point, but did not converse with and who were not posting anything I really needed. I'm following a few extra people on Buzz as it doesn't demand immediate attention like the microblogs. Buzz needs more ways to filter and priorities updates. Being able to group people would be good. I do this with Google Reader so that I can read posts on a given topic.
A discussion elsewhere was inspired by a friend who doesn't like using social sites as he feels they expose too much personal information to potential or actual bots that could pull together all sorts of data about us and draw conclusions about our movements, relationships and activities. I'm not as paranoid as him, but I do limit the amount of personal data I release. I don't talk much about family and usually only mention friends who are active on-line anyway. Others are posting every little detail of their lives on Facebook and Twitter. Either they don't care about the risks or just don't consider them. I don't really know how real the risks of identity theft are that you hear about in the press. I've played with semantic technologies like FOAF that make it very simple to harvest personal details, but also limited details of names, locations and dates there. It would be great to be able to build your address book from publicly published data, but it's likely to be abused. That said, I know people who have put their personal telephone numbers on web sites and not suffered from abuse.
I wanted to write more about how I was rationalising my networks, but I still haven't worked out the details. I don't follow hundreds of people, but I'm trying to keep the flood of incoming updates to manageable levels. I don't want to spend all my time reading them. I've got other things I want to do, like making music. I've been doing a few experiments with my guitar and uploading them to SoundCloud.