"Many people of a certain age (ahem) enjoy playing around with Facebook - but come away unsatisfied. Not everyone they know is in the network, and not everyone they know in the network relies on it to any large extent for communications with friends. Their college-age and 20-something children and nieces and nephews don't have that problem, because participation is almost universal. For that younger generation, Facebook is a one-stop shop for communicating and keeping up with all their friends, because everyone's in the network."
I agree. Everyone who is at school or college seems to be on Facebook; they use it to coordinate their entire social lives (as well as do various recreational, jokey things from what I can hear). It is cheaper than organising events or staying in touch by mobile phone when you don't have any disposable income. It isn't particularly my cup of tea, though I do have an account there, export my blog posts to my profile, and enjoy it when I connect up with a friend or colleague. (But I really don't "get" all these fans and pokes, and what do the groups actually "do"?)
Social networks for people older than teens or 20s are more challenging because most of us are in the "e-mail generation". (Richard Akerman has made this point also.) In the main, we aren't used to hanging out in a communal internet space. Even those of us who have had blogs for a while are somewhat hung up on traffic, comments etc to our blogs. A social network is a dedicated conversation space for exchange and discussion of ideas in a more open-ended, participatory way than a conversation on a particular blog.
I very much enjoy two social networks (as you may have noticed), Nature Network and FriendFeed. The former is a social network for scientists and for the hangers-on of science (like me). There are blogs, forums and lots of interesting and/or funny conversations, often with a scientific theme, but often not. It is free and anyone can join. Similarly, Friend Feed (a Google application) is free. It allows you to share anything onto your page or into a "room" there: a link, a message, an rss feed, your blog posts, your Flickr photos, Twitter, Tumblr, Delicious, and goodness knows what else. What's more, you can comment and converse on these links with your friends (you are allowed longer comments than Twitter, thankfully). You can easily set up a "room" for people who share similar interests. The people and rooms I have found there so far are mainly scientist-types, but I have set up a crime&mystery fiction room (please join!) and found a rather quiet book group room (please join!). Friend Feed is an easy place to exchange ideas and comments in a quiet, non-"shouty" environment, simple and clean. I hope to see you there sometime. My account is here.
If you haven't tried FriendFeed before, you can sign up and then connect with people, who can in turn connect with you. You can join "rooms" (groups) according to your interests. There are tabs along the top of your home page so you can choose to view your own page, a page with all your friends' links and conversations, or your "rooms". Worth a look.
Correction via Richard Akerman: "FriendFeed is from former Google engineers, but I don't think it's owned by Google". Thanks, Richard.