Kinda crazy that the magnitude is so small! (my next [admittedly rather cynical] thought is "who funded this?")
It would follow logically that whatever kind of brain rot social media causes, would affect 1% of the population very dramatically, another 9% somewhat more noticeably, and then there would be this vast ocean of people who are only marginally aware/affected. From the perspective of online activity they appear to not even exist.
This always seems counterintuitive to the 9% or the 1% (and just by commenting we're already in one of those demogs). But there's lots of data out there supporting these skewed distributions in online activity.
The business model of the casinos and the drug dealers and the alcohol venders is the same - you need a huge pool of unproblematic recreational users to find the problematic users who generate the bulk of your profits.
The same model works for video games and social media.
The casino, liquor store and drug dealer all make the same margin regardless of who they're selling to. If anything the problem users are more likely to cause problems for them so they'd rather make the money on casual users and scale.
Having your whole operation be basically a wash except for all the money from a few people with problems is fairly unique to digital gaming and the software industry.
Gambling is also very skewed. Studies place it something like 5% of in person gamblers accounting for 50% of profits or 1% for online gambling. I would guess for sports betting it's similar.
Essentially a disturbing way to look at it is that the people with alcohol addiction are allowing everyone else to be able to consume alcohol for cheaper than it would otherwise be.
Same phenomena exist for other addictive things like sugar in soda and free to play video games. (Although obviously soda and video games are nowhere close to alcohol in terms of destructive potential for those who develop an addiction).