Besides a couple of other LJs and a couple of GJs, I have accounts on most every major social network, with the notable exception of MoveableType, whose functionality I feel is entirely duplicated by Vox if not by LJ.
What do I do with them, you ask? Well, I periodically update Facebook with tidbits, and I'll get around to posting and annotating some photos. I use it primarily to comment on my students' and friends' pages. Beyond that, I view them as (weak) recruiting tools. My Facebook and MySpace accounts point people to LJ, as do all of my other blogs and social networks: Blogger, Xanga, Vox, Friendster, and Orkut.
There's my near-term plan, in a nutshell. How about you?