IMO it's similar to why Hinton works for Google - this gives him huge resources (data, compute, money to pay researchers) to do research with, unlike anything to be found in academia. Perhaps this is a naive view, but this is a guy who spent decades pushing for a direction in AI that was not popular but which he really believed in, so it seems natural he would want to accept resources to further research in that direction. Of course, he's also been public about thinking Facebook does more good than bad for the world, in his view.

Also, TBH I doubt he has much to do with the AI used for engagement optimization, his specialty is in other topics and he seems to be focused on the work of Facebook AI Research (which openly engages with academia and so on). And to be fair he is also still a professor at NYU and has students there.