Haven't looked into how accurate these are but it's mentioned fairly often.

"Market leader Facebook was used by 79 percent of internet users" https://www.statista.com/statistics/246230/share-of-us-inter...

"71% of online adults use Facebook" http://www.pewinternet.org/fact-sheets/social-networking-fac...

"72% of all online US adults visit Facebook at least once a month" https://www.brandwatch.com/blog/96-amazing-social-media-stat...

Given that Facebook boasts 1.8 billion monthly active users users, and there are about 3.5 billion Internet users, it seems broadly plausible that 80% of people on the Internet have used Facebook — but 80% definitely aren't regular users.

Pew Research had a January 2014 number of 71% of 'online adults' being on facebook, Statista had 79% of US internet users for April of 2016.

http://www.pewinternet.org/fact-sheets/social-networking-fac... https://www.statista.com/statistics/246230/share-of-us-inter...