About 10 million people in the US saw politically divisive ads on Facebook the company says were bought in Russia in the months before and after the 2016 US presidential election.
The social media giant, which had not previously given such an estimate, said in a statement on Monday it used modelling to estimate how many people saw at least one of the 3000 ads.
It also said 44 per cent of the ads were seen before the November 2016 election and 56 per cent were seen afterward.
The ads have sparked anger toward Facebook and, within the US, towards Russia since Facebook disclosed their existence in September.
Moscow has denied involvement with the ads.
Facebook has faced calls for increased US regulation from US authorities.
Chief executive Mark Zuckerberg has outlined steps the company plans to take to deter governments from abusing the social media network.
Earlier on Monday, Facebook said in a separate statement it planned to hire 1000 more people to review ads and ensure they meet its terms as part of an effort to deter Russia and other countries from using the platform to interfere in elections.