In 2016, Facebook published an article saying that 1 million people use Facebook (over their onion address) every month[1]. Comparing this with the privacy preserving statistics provided by the Tor project (based on extrapolating HSDir hits) leads you to believe that 1 million per month is the overwhelming majority of .onion site users.

Roger Dingledine mentions this in quite a few of his talks, I'm fairly sure it's an accurate statement.

[1]: https://www.facebook.com/notes/facebook-over-tor/1-million-p...