Users consent to Facebook seeing their traffic and it's suddenly a problem?
Users consent to the website seeing their traffic and the website consents to Cloudflare doing the SSL termination. This isn't too much different from the website consenting to analytics scripts monitoring webpage activity (i.e. Hotjar). If they did something shady, then users & the website would both be rightfully mad at them. But Cloudflare hasn't, so far at least.
Meanwhile, Facebook is known to do literally everything shady that is possible to do with a user's data, as well as plenty of things that weren't even a thing before they invented entirely new methods of tracking and selling data, so it's rightfully insane to trust them with anything, especially website traffic that they have no rights to.