MySpace did what you suggest. They monetized heavily, harming UX, and creating space for a cleaner modern user-friendly competitor. That's part of why they failed.

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.