Skim

To skim is to "take off the top". In general terms, it would be to remove something from the top of something else, such as removing leaves from a swimming pool or cream from milk.

In the porn business, and specifically with TGP and MGP sites, it means to redirect a portion of people who click on galleries to an alternate destination, for the purposes of traffic trades or to send the user to a membership site. At the extreme, the skim is all of the clicks made, which would then define the site as a circle jerk site.