You're right, I didn't produce evidence in my comment. Smoking and lung cancer are more clear cut than social media problems in general.

The effects of social media are more complex and nuanced than smoking. There are a lot of studies that show that social media has a negative effect on mental well being. When someones dies of loneliness or birth rates collapse and young people have less sex than ever, social media is never the only cause and might not even be the main reason, but it seems to play an important role.

Also, I don't think all social media is bad. I do love these discussions on HN even or especially when we don't agree, but tiktok and similar services have a lot of bad incentives with regards to user well being.