There's a difference between pre-loading content to improve user experience, and tracking mouse movements to gather information about users that's then used to show them advertisements, or potentially track them across the web[0].
There's also the difference between announcing "hey, look at this cool tech we have to make the web faster!" vs "we are legally required to admit that we've been watching you like a hawk, for... reasons."
I can't speak to Facebook's intentions or policys(I don't use big blue app at all), but many sites use tools like Full story to discover bugs or watch a customers user journey to discover flows that aren't working well. Some portions of the page are automatically filtered(inputs with type=password), and the rest depends on the team being very thoughtful about marking sensitive portions of the screen as such.
It's a very manual process, but probably one of the most powerful tools for improving user experience I've ever seen. And typically for most businesses, you are keeping these sessions for 2 weeks or thirty days at most.
I absolutely agree - I used something similar at my previous job.
But neither of us know Facebook's policy (although we can easily guess their intentions, based on past behavior), and nobody ought to be expected to cut them any slack.
There's also the difference between announcing "hey, look at this cool tech we have to make the web faster!" vs "we are legally required to admit that we've been watching you like a hawk, for... reasons."
[0] https://news.ycombinator.com/item?id=17301769