None. The big porn companies have direct banking relationships with (typically multiple - as banking access is typically the largest risk such a company carries) merchant banks. They will then use whatever payment processor they feel like (e.g. an authorize.net clone) or build their own, but hooked into their merchant account on the backend. These will (generally) be invisible to you, as all the API calls are done in the background and hosted by the company itself.

The small porn companies typically use ccbill or epoch, although my knowledge is slightly dated in that area.