Yeah keep in mind that the really popular websites don't use adsense. They sell their own advertising space for more than they can make using adsense. Most website owners like to a/b test all kinds of things though. I've tried lots of different ways of monetizing mine and have ended up back at adsense because it's been the best for me. Given that, I would definitely test facebook to see how it did and if it did better i'd keep it, if not I'd go back to adsense.