I tried Twitter for a while, but I didn't find most tweets that interesting. I also don't like limiting myself to 140 characters. It seems most useful for links to good articles online or websites or blogs. Not very conducive to chitchat or serious dicussion.
I was unemployed for so long that I got desperate enough to put myself on LinkedIn. I'm not very involved with it, don't check it regularly, don't try to build up my network. OTOH, recruiters have found me there, and I got some excellent advice from one. I suspect my LinkedIn connection indirectly led to my current contract job, so it was worth it.