The big strength Facebook has now is very similar to Google's. They can rake in cash without having to spend a bunch of money to generate it. Unlike Apple they don't need to produce extremely polished and innovate hardware every single year.
Secondly it was very difficult for any other party to aggregate as many advertisers as Google had; Facebook pulled it off but it took a long time. Someone else may be able to do that too, but once virtually all ad revenue is digital there will be plenty of room for a third participant. There are other business models, but the ad revenue one allows companies to really laser focus on just growing and keeping their marketshare.
AI and post-mobile platforms are going to change things in ways investors aren't predicting. Facebook knows this and they certainly are demonstrating it, between wit.ai/messengers "bots" and Oculus. This larger generational shift may well open up gaps that directly threaten Google's core model more so than Facebook's.