I've gotten rewarding freelance work through facebook referrals. I learned about, and conversed with, my future classmates over the summer before graduate school began this past September. Facebook gives me a way (and a reason) to communicate in a casual manner with folks I wouldn't otherwise keep in touch with. It's a great way to warm up a prospective client without resorting to a "pitch letter".
Facebook is a "take it or leave it" communication network. If I receive an unwanted email from someone I know, I'll either feel obligated to respond to it (which wastes everyone's time) or ignore it (which causes resentment). Facebook transfers the power from the sender to the receiver, making everyone benefit. If a status update starts an interesting thread, then YAY! If not, no big deal.
Facebook helps me realize which social groups are interested in what things. For instance, if I post an update about food, there's a nearly universal response. If I post about hacking my 1st gen Macintosh mouse to accept USB, I'll probably get comments mostly from other people in my (super geeky) graduate and a smattering of other friends.
Facebook creates connections I didn't know existed, and organizes my contacts into a system that makes sense. I've always been amazed at the handful of times when I've discovered that friends from completely different social circles have a friend in common with each other. You know, the whole "six degrees, small world after all" stuff. It's great!
Eventually, Facebook will no doubt become a historic "document" with countless future benefits. I can imagine retiring someday and going through the archives for source material when writing my memoirs or biography. All sorts of other applications will surely coalesce - ethnographies, trend-tracking...all kinds of stuff.
I (and quite a few other people) probably update my status around once every other day. Imagine if you started emailing everyone in your address book every other day. You'd be labeled a spammer so quickly your head would spin.
Facebook serves a purpose, however flawed it may be (500,000,000 is a lot to manage).
It's a series of tubes.