In my experience, the ads are the _least_ harmful parts of social media. It's the regular accounts that are causing FOMO and mental health issues by displaying unrealistic lifestyles that the majority of people won't ever achieve.

That's only one fragment of social media though, it's not helpful to focus on this part of social media alone; another big one is the rhetoric hidden in otherwise normal / legitimate content that seems to be a nontrivial factor in a hard shift to the right, politically.