I thought we are friends already
But seriously, I never bothered with those "networks". Seems to be too much work to keep up with it (not with the friends, but with the networks), as the popular ones seem to shift every once in a while. I get invitations every once in a while but usually decline for the before-mentioned reasons.
Only one(s) I am "signed up" for are Yahoo!'s 360ยบ (as it practically comes as a sidekick to the Messenger) and Google's latest baby "orkut" (which happened by accident somehow). I am not seriously using either one.