One argument is that for Facebook it's better that they own the big competitors rather than someone else (Google rolling it in to Google+, Apple doing something

One argument is that for Facebook it's better that they own the big competitors rather than someone else (Google rolling it in to Google+, Apple doing something with it, etc). As long as they keep swallowing up any "social(-ish) network" app before they become a real threat to Facebook, Facebook can still be the king.