Yes, you can argue social media is harmful, but it generally does not cause people to lose their jobs or spend all their money.
So, the assertion that social media is "addictive" would translate to, "it conditions the user to require a dopamine (or whatever) hit that they will return to the social media to acquire, and will feel bad (worse than before they used it) if they stop using." Whether or not that's true of social media is debatable, but I think it is raising at least a valid question.
On the other hand, even though Stack Overflow has a lot of the same software features, it has no such problem, so I think the important question is what does Facebook do differently than Stack Overflow to cause people to spend ever-larger amounts of time on it, to no real purpose?
Though even there I think they are moving to phrases like "drug use disorder" partly because of usage mismatches like this.
There's also the major difference in consequences. If you're truly addicted to social media, you're way less likely to destroy your life (although it could still be possible.)
If social media caused people to lose their jobs or spend all their money, far fewer people would use it, and public engagement with the problem would be greater.
I suggest that social media's harm to society is greater because it's less harmful to the individual than gambling, etc.
I suggest parasitism as a more accurate description of what seeking "user engagement" is.
By the way, are you arguing that smoking is not an addiction?
The definition of psychiatric disorders includes this kind of requirement. E.g. if you can't sleep more than a couple hours with no apparent negative effects, it's not a sleep disorder. On the other hand if this situation is unbearable for you and you feel you need to sleep more but can't, then you have a sleep disorder.
But yes I'd think this is a pretty loose definition that would include things like social media. Personally I think more is needed. Is that social media addict a depressed person who would just escape from life via an alternative means like by watching TV or something else? If so we should call their problem depression, and call social media simply an "escape" not an addictive behavior.