I get the impression that they periodically get a report of every winning bid that is made via those commissionable links, that actually tells them how much money that they are getting from bids made via those links. Now of course, if your Priceline handle and your BB and BFT handles are completely different, they would have to make educated guesses as to who is posting about those bids - but it may specifiy the winning bid price and the venue, since their commission is some small percentage of the winning bid price.