Advertisers have a hard time trying to advertise to the right demographics. Anyone who's worked in any business that has an advertising budget (pretty much any business) knows that setting up advertising campaigns is like throwing a dart in the dark at a dartboard a mile away.
With social networking sites, everybody will give you their information – with that kind of information disclosure, there's almost no need for research teams!
Of course, there's the whole pedophile stalking issue, but without them we would never have those entertaining Dateline NBC series. As long as people aren't stealing identities and slapping their watermark on them, we should be fine.