How much you let people know about yopu depends in you and only you.

I have been dragged to most of the social networks I joined. Included Facebook, however, only after I've tried them I can tell which ones I actually like the way they work and I have the people I want to be in contact active enough for me to stay.

I liked tagged a lot, but my friends didn't, so I ended up deleting my account, I also deleted many of other portals I joined because of the invites that wouldn't cease to fill my inbox.

I don't think you must be obligated to have facebook or myspace or twitter just for being on the netz, and I don't think you are losing much, specially if you are the kind of person that keeps in ontact through writing messages in emails and using IM.