With some 700 million users, Facebook is one of the biggest forces in the tech world today. But their glaring weakness is that they do not ultimately control their own destiny. They have flourished on the desktop-based web, which is mainly open, but mobile is the key to the future. Facebook has been doing pretty well here so far, but because they do not control the platforms they are on, things are likely to get hard for them going forward as rivalries intensify.
Which is why they are always rumored to be creating their own phone.