We tried recently tried relatively identical campaigns (few grand into each) for AW and FB for http://premium.wpmudev.org - AW was by far far the winner... can't beat people actually looking for stuff, rather than annoying them while they're trying to socialize!

In my view, that's where FB falls over most of all, they're only fundamentally attractive to agencies looking for 'in your face' rather than 'purpose based' ads... pointless for a highly niche relatively small budget crew like us.

And why goog will kill them in the end :)