Seems like the desired outcome here is an unfixed target. What does Facebook want to show you next in the Newsfeed, and why? It's gonna be different from the factors Amazon evaluates to determine your home page suggested items. So while it's worth trying to replicate academic research on these datasets like MovieLens, the real-world outputs from collaborative filtering are inherently subjective.
In general, if you provide a social media service, optimising for ads will probably not lead to you becoming a world-dominating destroyer of democracy, like Facebook.
You probably want to optimise for keeping people on your service, first (and last).
Yeah, at least in the short term. Which makes me think that beyond obvious patterns, the question of accuracy (is this a successful recommender system or not?) is hard to define in a way that can be demonstrated in a lab.
