I really appreciate your response, which made me realize this is more nuanced than I thought at first blush. (I wrote that reply before you linked the National Academies review, which was quite helpful.)
Maybe I am truly morally panicked, but I'm really hesitant to brush aside the evidence as "basically nothing." Small effects are worth paying attention to (https://psycnet.apa.org/record/2022-26026-014). And quantifying the harms of social media use is less like quantifying the harms of, say, smoking cigarettes, because social media has (indeed, is built on) network effects. You could plop a kid from the 50's into a modern American neighborhood and their mental health might decline even if they don't use social media because of the way it has changed childhood. When the social life of an entire generation is transformed by technology, it stops making sense to ask "How much does one hour of Instagram hurt mental health?" Yet that is essentially the question all the studies are designed to answer, and even still, we see an effect.
Anyway, thanks for taking to time to explain your perspective.
IMO the correct framing most supported by the evidence is that a few vulnerable populations (most likely within a certain age bracket, and already dealing with other significant life issues) are definitely at risk of nontrivial harm from social media. Otherwise, for the majority, it is likely a wash. And since the bias in studies is to look for negatives (much like drug studies don't properly measure negative side effects until much later), we also can't say if there aren't subgroups benefiting hugely as well (e.g. autistic people that can connect online, or other people with unusual interests that before would have remained isolated and disconnected, feel crazy, etc).
So, yes, small effects can be important. The observed effects for social media are quite small and consistent with being driven solely by a small, vulnerable population, however, and this make broad social injunctions and moral panic less defensible.
Also, these are effects on self-report or other psychological instruments, where in general you need to find something like a minimal clinically important difference before you can determine if you care practically about these things. E.g. if a standardized regression coefficient of 0.1 translates to a 1-point shift on a 20-point life-satisfaction scale, is this even something I (or anyone) can even notice? Does it even rise above measurement error?
Since no one is doing that kind of research here, we just get these (relatively meaningless) standardized effect sizes, and it is really basically impossible to know if we should care at all about the observed effects. This is mainly what I mean by the evidence being flimsy / weak, it is really too inconclusive to drive decisions at this point.