Yes, as OP mentions, machine learning can be applied quite well in other fields "like image processing or winning at strategy games."

And yes, they test out their new ML algorithms to see how effective they are. But they don't need the machine learning to create their list of "gullible people." Rather, they don't even -need- that list. If you just serve stupid, shady ads that look as close to porn as that platform can get away with, you'd get your desired click-through rate. The actual personalization aspect is just a myth to validate our invasion of privacy.