I think the population you're observing here are people who first joined a FAANG before 2014 (since I imagine most people stayed for their initial four-year grant given FANNG stock performance). Facebook (not sure about the others) in 2014 had ~2k engineers IIRC, but Google IIRC had like 10-15k engineers around that time. The gap has closed in recent years, but looking at current engineering numbers isn't the right approach IMO.