Based on the census report on education, from 1970 to 1999, the number of institutions (2, 4, and both), instructional staff, and the total enrollment have doubled or nearly so (number 257), as have school expenditures (number 199). Over the 1990s there was an approximate 3% annual increase (population grows at a bit less than 1% annually) in the instructional staff. That said, the number of graduates was quite high in many fields in 1971 (number 279) - quite the baby boom - and since then the social sciences (and humanities, but not splitting out economics, which others may suggest has grown since 1971) have not fared so well, seeing a decline in the number of and graduates (with business, communications, biology, health, and even parks and recreation seeing sharp increases).
My feeling is that starting your training to become a professor in economics in four-six years is a good, not great idea. It is even better if you choose a field closely related to economics such as business (or even finance for you EJMR junkies) or do economics of education, urban economics, health economics, etc and be open to hires from hybrid departments.
As for the particular question about the national make-up of professors, From the professors in the US that I've known, I do get the impression that American born professors are more likely to be older, and younger, even under 50, professors are more likely to be US born, which reflects a trend that seems to stretch back around 25 years. That is, I would guess that the shift to fewer US-born graduate students started around 1985, and has accelerated since then. Even Clark medalists are now regularly foreign born (3 of last 4, Acemoglu, Saez, and Dufflo with Chetty waiting in the wings). That is, I don't think the growth of international institutions - particularly in East Asia, has greatly reduced the allure of academic positions in the US - the changing nationality of PhD's in economics is directly reflected in the changing nationality of your professors (a welcome trend, I think).