Some large tech behemoths could hypothetically shoulder the enormous financial burden of handling hundreds of new lawsuits if they suddenly became responsible for the random things their users say, but it would not be possible for a small nonprofit like Signal to continue to operate within the United States. Tech companies and organizations may be forced to relocate, and new startups may choose to begin in other countries instead.

Other sites could absolutely spend time crawling for new lists of breached passwords and then hashing and comparing against their own. However this is an intensive process and I'm sure both Facebook and Google have a team dedicated to account security with functions like this.