I've argued before that any closed platform will do whatever is in their business interest, irrespective of whether it hurts developers in the process. This is true with Apple and the recent 30% outcry, and it is now true of Facebook, who has fairly understandably decided not to give a competing advertising platform access to its users.
It may suck for Facebook developers, but it was an entirely predictable move. Facebook and Google are both in the business of advertising. They are both competing for the internet top spot in terms of attention and advertising, and Facebook, being the underdog in this fight, will do whatever they need to beat their gigantic rival.
If you build your business on the intersection of not one but two closed platforms, which are in fact competing with each other, you really are doing everything you can to end up being a collateral casualty in the war of the titans.
The HN discussion is currently just taking off but there will probably be some interesting insights here.