What a weird way to approach that claim. The population inside or outside the USA plays a tiny role. a) China and Russia aren't involved, so strike 2bn from that b) it's not about people "existing", it's about people being a customer (or product, whatever view you prefer), facebook is obviously more widespread in a first world, high tech society where practically everyone has a smartphone than in some thirld world country where electricity is rare and unstable and c) revenue per user. statista [1] claims it's much, much higher than in the rest of the world. With that coming in at a 3:1 or 4:1 or even 5:1, that alone would make the US a heavy weight in revenue.

[1] https://www.statista.com/statistics/251328/facebooks-average...