In no particular order…
Twitter does way too much for me to classify.  I keep up with friends, get news, find links, have my questions answered and meet new people.  Still my favorite.
I use FriendFeed as the place to share the things I find and write.
Just started using the TC50 winner to keep better tabs on what our team is doing seeing as we’re geographically challenged.
Willow Glen 2.0 is the social network I run for my neighborhood.  It’s a good place to see what’s happening in my ‘hood and meet my neighbors.  My go to for local info.
Facebook went through a down time for me for a while, but recently there have been a ton of old high school friends joining up.  It’s good for catching up on long lost relationships.
Not really a social site, but I subscribe to 78 blogs.  There’s no way I’d be able to keep up without it.