Yeah ha ha "simple" in every sense of the word. You can toss in another "simple" app by the name of Flickr.

they are relatively simple in terms of business logic. As long as you can 'shard' a problem parallel servers can solve your problem easily. As soon as that is no longer possible because of business logic constraints the real fun begins.

They are really simple. Try comparing them to some enterprise solution if you had a chance to work on one.

its not exactly quick and dirty...its rapid development, which means they eventually can get something very complex (which I'm sure facebook has in place), but if they need to start over, they can do so quickly.