Facebook is too ephemeral, and of course, has the worst possible policies towards user privacy - something LJ figured out ages ago. By ephemeral, I mean - people's posts are gone in a few seconds, because they're buried under the dross of farmville, which-pop-culture-numbskull-am-i, mafia wars and other pointless drivel.
I miss reading what my friends are up to, working on, and thinking in a substantive way. We're left with "I just had a ham sandwich!" or "Off to [some show I've never heard of]" and drunken pub pictures.