With LJ, I only friend people who post often and in a style I like, because otherwise I'll just end up skimming their posts and I'll feel like I'm lying to them by having them on my flist but not reading them. But I friend most everyone who requests it on facebook, because I don't have the same goal there. With LJ, I want to develop real friendships; with facebook, I just want to keep in contact and get to know people. I'm open to developing friendships via facebook, of course, but I don't feel I'm setting a goal of friendship by adding someone there.
Facebook is so different from all the other social networking sites I've been part of; I'm used to internet communication being pretty exclusively with not-in-person friends. On facebook I mainly connect with people I've known in person; relatives, in-laws, high school friends, friends from so long ago that we disagree on most everything, and (since summer) local friends. It's so weird to me! And I never thought that such a 'superficial' method of communication would have any value to me, but I actually have become quite fond of facebook. I've found that even when people don't share lengthy introspection, I can get a sense of who they are and what's important to them through their status updates, links, and photos. Not as good a sense, of course, but something meaningful nonetheless.
Twitter is another 'silly' method of connection that I've become fond of. There I connect with LJ friends, mainly, and I've loved seeing a new side of them -- all the little things that aren't enough to post about but still interesting enough to share. I have it connected to my facebook status, as it serves pretty much the same purpose but with a different group of people. And it's become kinda an art form to me to make tweets that are exactly 140 characters and neither fluffed nor missing important info.