Links for the sake of links are useless.
I link to other blogs because I find them interesting, and I don't want links back from blogs that don't interest me, or from people who don't find my blog interesting.
It's the same with linking to individual posts. I'll link to a post I find interesting, and I'm glad when other bloggers link to posts in my blog that they find interesting, and people who read their blog may find my blog interesting.
Ity's the same with people who want to be your “friend” on social networks like MySpace, Facebook, or whatever, just to add to the number of their friends. If they haven't at least exchanged a few e-mail messages before, I am not interested in being their “friend”.