I don't expect logic to follow "think of the children", but what's the argument for a marketplace? So criminals can send pornography to each other b/c it's a communication platform. But how do they use facebook to exchange money for pornography? As far as I know you can't buy digital media on Facebook?

If people are just using it as a medium to communicate and then doing deals offline or through other channels then it's about as absurd as holding the phone companies accountable for all deals arranged by phone