In aggregate it would be hard to say. But at the extremes I think you could argue that social media is worse. For example, getting severely depressed and killing yourself as a teenager due in large part to a distorted sense of reality from social media is probably worse than getting esophageal cancer and dying in your 50s. Both are extreme outcomes of a bad habit, but one is way worse in my eyes.

Maybe. Smoking is a very social habit at least. Whose to say the isolation and depression from phones/social media is less harmful?

How is smoking a social habit? I see people stepping away from groups to go smoke elsewhere.

Back when more people smoked you'd see smokers congregate and chat while smoking, often smoke breaks would be coordinated. It still happens, but less often because fewer smoke, especially in professional environments.

That said it's pretty rare I ever see a single smoker off by themselves, unless they've gone out of their way to be alone.

In it’s heydey those people stepping away were a group

If I absolutely had to choose a vice here, I’d choose smoking for 20 years over living in social media.