Facebook doesn't cause cancer of course, but there's no shortage of evidence that it harms some of its users, as well as contributing to social phenomena that harm non-users. The Rohingya genocide is a prominent example.

https://en.m.wikipedia.org/wiki/Facebook_content_management_...

Hard to compare, in terms of what is worse, cancer or depression but I am also surprised more people didn't write here.

https://www.apa.org/monitor/2022/03/feature-minimize-instagr...