I am "on Facebook", but it's for four specific reasons:

1) So I don't get questions about why I'm not on Facebook.

2) To let friends tag me in pictures (and see what pictures are being posted of me).

3) To let friends invite me to parties through Facebook.

4) See pictures of friends' babies.

I think it's good to have a Facebook account. It lets people push stuff to you, and more often I'm seeing invitations coming exclusively through FB. But I personally don't think you're missing anything by not actively using it.

Likewise, LinkedIn can be nice to keep tabs on past colleagues, not those you were close to obviously, but those on your periphery. You don't need to participate in the community to get the biggest benefit, IMO.