1) While they're not perfect, I'm not sure I believe they're doing more harm than good. As an annecdotal example, my family loves Facebook, because I live pretty far from most of them, and they like the ability to know that I'm healthy + happy, and/or if I'm travelling that I got back safely. They're generally older, so not that tech-savvy, and I do give Facebook Design a lot of credit for making some really intuitive choices in a lot of areas that make it accessible to older / non-geeky audiences. Could I just text / call them? Sure, but I'm a busy person like most people here, so it's easy to just throw up a picture or post to FB and make their days a bit better.
2) A lot of the arguments that they're doing "harm" are pretty pseudo-scientific, if you look at the claims/articles closely. Many rely on "correlation over causation" views, anecdotes, and the like. It's only recently that some true studies have tried to prove this [1], but even then arguing for putting FB on the level of tobacco and guns is a strong claim to make off of minimal peer-reviewed evidence. We might as well toss Google + Amazon + Twitter into this same boat.
[1] https://hbr.org/2017/04/a-new-more-rigorous-study-confirms-t...