I can share some input here
There will always be escorts/agencies and ofcourse fake profiles trying to weasle their way into any dating site (particularly adult dating). What portion, all depends on how the site operates.
IwantU, for example, employs real people whose sole job is 'browsing'. These people must read and approve every profile submitted or modified on the website. They then either accept it or reject it (there are many reasons to reject it too: wrong club, underage, invalid profile, spam).
This means we try and stop as much of the fakeness as we can. Obviously you cannot stop it 100%, because some people will create profiles manually, putting average pictures and normal text (they spam you once you have sent them a message). Here we just try and find patterns, and scripts notify us of things like many profiles from the same IP, etc...
Now on the flipside, some sites don't check anything at all. I can give you one perfect example:
I signup and post my profile as junk. 'ksdgkfjsdgf kjsdgkjfs dgkjf' will do nicely. I get the message my profile is pending and it will take upto 24 hours, that's pretty normal. Two minutes later I have an email saying my profile is approved, and then I start getting mail saying people read my profile and think we click, etc etc.....OBVIOUSLY script-generated messages from the site itself.
So you can see the number of fake profiles you find will vary depending how the site is run. I can tell you though that there ARE hot women who use dating sites these days who do just want to fuck
