I don't see any abuse here, and I believe that their lack of transparency wrt. filtering algorithms is justified.
First off all, the shortest description of what they do wouldn't probably be far from publishing the algorithm iteslf. An algorithm that's ever changing and probably different depending on where you live or to what group you were randomly assigned. 99% of people wouldn't care anyway, and being transparent about the algorithm would likely make them less happy - right now they accept Facebook as is and don't think twice about it; give them the description of how things work and suddenly everyone will start saying that Facebook filtering sucks because random-reason-511.
Moreover, the only people that stand to benefit from knowing Facebook's algorithm are advertisers, who will game the hell out of the system for their own short-term benefit, just like they do with Google. It's something neither Facebook users, nor Facebook itself want.
