Facebook Inc. on Monday said it estimates 10 million people saw ads it has discovered on its platform paid for by Russian entities, but warned that it may not have uncovered all malicious activity that attempted to interfere in the American political process.

The revelation from Facebook quantifies for the first time the spread of the known Russian activity since the social network said last month it had identified 470 “inauthentic” Russian-backed accounts responsible for $100,000 in advertising spending. Facebook on Monday...