I think FB can be bad for mental health from the perspective of their administration or mods or what have you, being very slow or completely negligent toward addressing or removing abusive types of posts and harassments, bullying, even bugs in their software. Some of the most simple problems aren't even fixed. I'm a bit slow with providing examples here but trust me, what I'm saying is correct, and it drives you nuts. So, yes, very bad for mental health - in some ways.