Never forget that the internet is awash in dead companies and sites that were extremely popular and powerful, until they werent.
Facebook is a giant, but its dominance in the USA and the EU is likely short lived.
It is not competition that will kill Facebook, its the change in people's behavior. Very different from the competitive view you take.
The death of Facebook will be from both in how people interact with each other and tech. Whether its a fundamental shift in comm(mobile phones) or a leap in tech like serverless / peer to peer that removes the need for a central service. Eventually, Facebook will not exist anywhere near its current form.
Remember, FB not a service like Google that serves the user. You dont need them to do anything but connect to other people. If you dont need to use FB, you wont. For now, its easier and working very well at scale. But that's temporary. Maybe 10 years. Maybe longer... One day it too will be a memory, owned and operated by a PE firm that specializes in brands.