I used to work in the space, assembling the DB table you described in a meaningful way is surprisingly hard to do, because you would need to attach confidence intervals to everything you tag onto that user_id. Reality tends to be complex and often wrong, and there are very few actually good signals. That's what makes Facebook such a powerhouse.

I eventually left that space for personal reasons, but with distance and time I've resolved that I no longer want to touch that space, no matter the startup idea. I would sooner start developing software to block or push noise into those systems. The digital ad/marketing industry is driven by a lot of people who care about anything but the health of our society or the actual consumers.