Facebook only has the pick of engineers they can convince to move to California and work for Mark Zuckerberg. I know it's hard to believe, but there are a lot of us who have turned them down on multiple occasions.

The competency of FANG engineers in general is highly overrated. They are humans. They don't have access to some kind of forbidden knowledge. The idea that these companies are uniquely competent such that competitors don't even have a chance is largely a marketing story.