I live in Seattle, one of the hottest housing markets in the country. If I go to Airbnb and select "seattle" "entire place" "house" and "flexible" it comes up with 836 options. SF had roughly the same 700-800 (and a few from the larger bay area bled in).

That doesn't seem very significant in cities of roughly 750K/875K (within city limits of Seattle and SF respectively)