“If you care about your online presence, you must own it.” Fred Wilson discusses how to do that. To his advice I would add “be consistent.”
I don’t try to be everywhere all the time, but I do try to post a few useful things to my tumblr each week. I use tumblr and twitter for my “work” related, public posts, and Facebook for my personal posts. I deliberately try to keep the two separate so that I can prevent my politics from affecting my professional life, but mostly so that I can avoid annoying my non-geek friends with geeky things.