Google scholar is the easiest source, but I can help by sharing a few (there is no single paper I know of that sums everything up). It's a difficult topic because so few solid studies have been done - but everything we know suggests that its effects are - on average - weak. In other words, online time appears to neither add to nor detract from the lives of most people (at least directly). It's just time filled up with vanilla.

See: https://www.sciencedirect.com/science/article/pii/S2352250X2...

And: https://www.annualreviews.org/doi/abs/10.1146/annurev-psych-...

https://europepmc.org/article/ppr/ppr407158

But there are a minority of folks on either end of the spectrum. On the negative side, specific types of personalities, attachment styles, and so on can suffer significantly on the internet and especially on social media. The media loves these studies, there’s a ton and they are easily found so I’ll just refer you to scholar.

Then there are groups of people that gain from the internet. The few studies that have bothered to investigate how people use their time online and in social media in detail have found that talking to internet randos in one-off fashion does not help, but that - and I’ll quote here:

“When people use social media in a truly social manner (i.e., actively interacting with meaningful social relations in a way akin to in-person social interactions) it was positively associated with psychological well-being. We propose this is because truly social usage promotes meaningful social relations, which result in positive psychological consequences such as reinforcing one’s identity, feeling valued, and mitigating stressful situations. Yet, when people use social media in other ways (e.g., passively engaging with weakly connected others, celebrities, brands, companies, or strangers typically for entertainment purposes) it does not influence psychological well-being. Therefore, how and how much people use social media has implications for their psychological health.” (in https://www.msi.org/wp-content/uploads/2021/05/MSI_Report_21...)

Of course this is but one study, and an as-yet unpublished one (though it appears well-designed). A meta-analysis from 2017 using this as a frame though backs this up here: https://journals.sagepub.com/doi/full/10.1177/09637214177308... - and a more recent though poorly written one generally mirrors it: https://www.frontiersin.org/articles/10.3389/fpsyg.2021.6787....

Another recent study - this time on the 60+ crowd - found “answering questions online were positively related to depressive symptoms” and looking at photos of non-family members on social media was associated with anxiety (https://journals.sagepub.com/doi/pdf/10.1177/146144482110543...)

Then there’s the work of Alexander J. A. M. van Deursen, in which he focuses on who exactly gains from their time on the internet and how - and how this in turn perpetuates inequality. You can find his work here: https://research.utwente.nl/en/persons/alexander-jam-van-deu...

But the foundation of the argument for me lies more in the research on what contributes to happiness and well-being, and which of those variables can be realistically gained from time on the internet and social media. What in the long term will make a difference in a person’s life? And invariably the most impactful outcome is the formation of an offline friendships and so-called "social capital". Which appears to generally be a rare thing - https://www.tandfonline.com/doi/abs/10.1080/1369118080263543... - though it does happen, and it seems self-disclosure is a key to that (among other things) - https://journals.sagepub.com/doi/abs/10.1177/146144481985874....

There’s more research that can be brought to bear on this, but that at least may provide a start for your own inquiry.