One of the major problems with Facebook, in the past, has been its one-size-fits-all privacy settings. You could decide, once and for all, who sees your status updates, but could not do so on a post-by-post basis. Which is fine and dandy when all your friends there know you in the same context, but becomes a problem once you have people from different spheres. You might not want to bore your generalist friends with detailed discussion of your more specialised interests, or share your personal life with your coworkers, so the only option is to self-censor down to the lowest common denominator, and hope that those who want more can be bothered with LiveJournal.
Not for much longer, though, because Facebook are soon rolling out post-by-post privacy options, which will let you decide, with each status update who sees it.
The options will include "everyone" (i.e., anyone who goes to your public Facebook page), friends, friends and friends thereof, or, most usefully, custom groups of friends. The devil is, of course, in the details, but it looks like it will make Facebook a lot more useful as a fine-grained social communications tool.
When they do it'll be about bloody time too.