I work in online advertising and I would agree if it weren't for the number. But, Facebook are selling ads. They probably still have a lot of low hanging fruit to pick. Advertisers have probably been getting a decent bang for their buck because it hasn't been the kind of economic climate that's very friendly to frivolous ad spending.

The only "theory" I can think of that would explain Facebook ad revenue as a temporary phenomenon is businesses trying it out or trying to get in early in anticipation of the ad platform improving. I don't buy that though, not at this point. I wish I understand the economics of it better.