http://www.byrnehobart.com/blog/a-clever-adwords-hack-how-to...
When people run ads pointing to a site they don't own, it's often in an effort to get an implicit endorsement from that site. Note: I have no evidence that this is going on, but it is a risk with any site that has a recognizable URL and allows user-generated content.
From what I can see, it looks like ads pointing to FB were a) big brands advertising on their own brand names in order to promote social sites (e.g. 'windows 7' or 'hennessy'), b) big brands advertising on generic terms (e.g. 'hummus' ads taking you to the Athenos Facebook page), or c) house ads for navigational searches, including one for the notorious "Facebook login".
These ads are also growing in popularity. It looks like they doubled between April and June.