My guess would be ads.
@Lexi - with all due respect, some of titles for your ads just aren't very good. I get what you're going for, trying to be provocative and all that but a lot of them are a little too random/vague/maybe even uninteresting and often have little to do with a session. Like "all done at the gym". Ok...but that's not overly titillating. "friend in from out of town and need some cash". That one bordered on sketchy.
The other thing is that your ads don't have pictures. I imagine this is known and without the instant gratification of opening a thread and seeing a pretty lady, why open the thread, particularly when it has an ambiguous title?
Lastly, someone else had mentioned but some SPs just bump the same ad once a day instead of posting a new one. THATS how you get a crazy view count more than anything else.
Best of luck