I think they're just waiting until they've completely solidified their #1 position to concentrate on making more money. It will seem obvious in hindsight that they make $20+ billion, the way it does for Google now. It may never be quite as good as Google though, but max potential is especially hard to predict.

True.

But if you are constrained by your team (ie, you can't work on everything at once, which Facebook says is their biggest constraint) and you have plenty of easy money from investors, why would you try and profit now and risk another service taking your audience when you could possibly build something that can profit for years to come?