As you said, Facebook has 500+M users, among those you'll find people annoyed by bugs and willing to: "to simply drop the service altogether and invest my time

As you said, Facebook has 500+M users, among those you'll find people annoyed by bugs and willing to: "to simply drop the service altogether and invest my time elsewhere." But obviously those users are a tiny minority. I challenge your conclusion that they should get their priority straight: if their goal is to keep growing at current rate as you seem to suggest, then they probably shouldn't change a damn thing, because they are darn good at it.