Hi Paul, I started to use Facebook and Twitter to promote my sites about 6 months ago. I have created a Facebook page for each models and used the category "entertainer'. You have to use a different email address, user name, password etc for each one because Facebook recognises you if you don't.
This is an example:
https://www.facebook.com/pages/Louis...87732444704781
I post a new pic every week and the word soon spreads.
Likewise on Twitter I have an account for each website and post regular pics.
example: @nylonsnylons That one is quite new and I have about 900 followers.
It is difficult to say what level of business it all brings in but it helps to keep business going when normal website sign ups are down.
I hope that helps. Give it a try. It can't do any harm.