The expected /. reaction to any story about Facebook, already evident in this thread, is along the lines of "Donotwant", "Lame", "Privacy Fail", "When I want to talk to my friends, I pick up the phone", etc.
Two facts I think slashdotters overlook: 1) "regular people" (i.e. everyone else) *love* the epic load of crap pretend socializing that Facebook provides, and care very little about the security of their information. b) there are a lot more of them than there are of us.
Google spends all of its time trying to mine your info (as a byproduct offering some really useful services - unlike Facebook). So they care very much about the "Social Web". Facebook has a half billion people tripping over themselves to cough up their personal info and build the Social Web basically for free. Honestly, if Facebook had a good search engine & email client, a lot of people would probably never go anywhere else. Sounds like a legitimate threat to me - even if not a single one of them can fix their own computer or speak Klingon.
If anything saves us, I think it'll be the fickleness of the mob. Hopefully, someone else will come up with the next Big Dumb Thing with Extra Farmville!, and Facebook will lose its grip the way MySpace did. But I doubt it'll be because 500,000,000 people suddenly wise up and realize they're not "really" socializing.