How do you know that ?
"paradoxically the most popular website for Tor users is Facebook "
How is it possible to make that affirmation when you don't know what Tor users are up to and Tor prevents metadata from being used ?
I just went through Tor's Overview page and it specifically states :
Using Tor protects you against a common form of Internet surveillance known as "traffic analysis." Traffic analysis can be used to infer who is talking to whom over a public network.
So you don't know who is talking to who. To me, that means you don't know if Facebook is the preferred web site for Tor users because you don't know what other websites they're using.
Also, using Tor to post your life on FaceBook. Now that's ironic.