I've been using Twitter for quite some time now, I'd guess about 2 years. I find it really useful for picking up on what issues are being discussed in libraryland, finding out about must-read blog posts and getting advice and ideas from my professional network. It's also a great way to feel part of a community, as the discussion is not all work work work, and often I get involved in more personal discussions, or things which are just plain funny. I have seen the #libchat stuff appear in my twitter feed from time to time, but Wednesday's are not a good day for me to be doing extra-work-work so I haven't been involved in this. (Have now started following @nataliebinder though)
I have to admit, I've never heard of it and I am not that interested in the concept. I use Delicious to keep a track of my book marks and I use twitter and stuff to find out what other people are reading. I don't think I need yet another social networking account to use up my time. Sorry - I haven't even registered.