https://www.forbes.com/sites/quora/2017/07/31/is-working-at-...
This Wikipedia entry says there are 3.87 million professional software engineers in the U.S.:
https://en.wikipedia.org/wiki/Software_engineering_demograph....
So if we're generous and say FAANGs employ 100K engineers, that would be about roughly 2.6% of U.S. software engineers.