I'm on Facebook (for fun and socialising, finding out what my recalcitrant nephews are up to and playing games), MySpace (to have a shop window for some of my songs) and Twitter (although I haven't done anything much with it).
A profile page in Mudcat would be good for all sorts of reasons.
If I'm following a thread I'm quite capable of checking in when it's convenient - I don't need further notifications sent automatically.