I heard that the site keeps 50% of the take, which is a huge amount for them. I saw Dillon Anderson post about a year ago he was making $15k / month on OF. Hes probably north of $20k now.
In part this market was created by the sub-par product the studios have been putting out. Many of the vids on the fans sites the guys plan, shoot and edit themselves. The product can be a bit amateurish, but the sex is hot AF b/c it's not being staged by or interfered upon by the studio directors and photographers - it's basically hot guys having hot sex. No incomprehensible or forced positions poses or plot lines. They self-select the other guys they want to perform with.