Let's get one thing clear: If social platforms wanted to protect children from abuse, they could have done it anytime in the last decade. No new laws are necessary. If they can build a buyer's profile and link it to a real person, they must definitely know which users of theirs are very likely to buy child #porn.
We must therefore assume that it is simply more profitable to sell the information to those who can offer such products than it is to block potential buyers and report them to the authorities.