In my opinion, there is nothing wrong in wanting to be exclusive with someone your sleeping with. If things lead in that direction, there shouldn't be a problem in discussing the options about your relationship and sharing with her how you feel. If you feel once you sleep together that your exclusive than share that with her. I'm quite sure she would feel the same way. Just because she's still on that site doesn't mean anything, and it could very well be due to boredom. I would just go with the flow and be honest with her and tell her how you feel.
If you can't handle the thorns, don't crave the rose!!