Scale may have something to do with it, but I don't think it's intelligence. People at Google are not sitting around complaining about the idiots they have to work with. That happens almost anywhere except Google, but Google has had pick of the litter for years now selecting heavily for intelligence.

As an early-stage startup guy and hired gun, my outside perspective is of course biased towards the smaller company, so take this with a grain of salt:

I think the main thing is that Facebook has a unified product and vision, with really strong UI, and a lot of hardcore infrastructure all centered around a global experience. Meanwhile Google has one strong business, and is repeatedly throwing spaghetti at the wall to find new areas to expand to. Sure there's a lot of great products there, but nothing with the potential Facebook has.