This is according to data released by Google's ad management and ad serving technology platform, DoubleClick, for its network of partners.
DoubleClick also reported that Facebook users boast a high level of online engagement.
The average visitor goes through 1,150 pages on Facebook per month. With businesses expected to spend more on social media marketing, this suggests that brands may want to make the focus of their online campaigns. Facebook currently has fan pages for brands, companies and businesses.
Similarly, Facebook had a 46.9% reach among all the web surfers tracked by DoubleClick.
According to Google data, Facebook had about 870 million unique visitors in June. Facebook reports that it has over 750 million users. Techland has suggested that the discrepancy is likely due to visitors who do not have, or weren't logged in to, Facebook accounts upon landing on the most popular website.
Second on the list was YouTube, a Google-owned website, with 790 million unique visitors and 100 billion page views for the month of June. YouTube measured a 42.6% online reach.
Google's rival search engine, Yahoo!, came in third with 590 million unique visitors and 78 billion page views, with a 31.8% online reach.
CEO Mark Zuckerberg said in July that the company didn't publicize the milestone because user numbers are not as important as they once were.
Unsurprisingly, due to its invite-only policy and recent launch date, Google's social network, Google +, did not rank within the top ten.
Also on the list are:
5. Wikipedia, 460 million unique visitors, 6.4 billion page views, 24.5% reach