Not just "work for" but "have ever worked for" which is, at this point, a much larger number.
What percentage of developers that is I haven't the faintest clue. But definitely significantly more than 10% of my friends have worked for at least one of FAAMNG. And I have a lot of non-nerd friends too.
The propinquity is insane. When my GF decided to leave MS she walked from her MS building on her lunch break to her interviews at Google and LinkedIn (LI has subsequently moved). Of course the inverse is true too: there aren't many startups in that area, except in the Landings complex (where Cygnus used to be!).