That is a really good point. There is a certain kind of nationalism/exceptionalism inherent in the US deciding that Facebook is both too essential to be left to its own devices and that naturally the US should decide what its policies end up being for the rest of the world -- and I often forget that perspective because I'm in the US and to a certain extent guilty of forgetting about the wider implications of US policy sometimes.

On the other hand, a robust market has fewer of those problems -- the US deciding "we don't want a US company to control the entire market, and we want more companies with differing policies" doesn't have the same implications as the US deciding, "we want the US company to do what we want everywhere." So there's a lot less exceptionalism rolled up in the idea of subsidizing alternatives; and non-US governments have already started to talk about either subsidizing some alternatives like Matrix or adopting them internally within government departments.

Unless the idea is to only regulate how Facebook handles content being displayed to US users, but (while companies do often have country-specific policies), drastically increasing the scope of that kind of system has a lot of its own implications about filter bubbles and communication between countries.