I think your experiment was valid, even if anecdotal. This article from January 2009 was talking about the phenomena of what it actually meant to have friends on facebook. Are you a "loser" or a "social slut"? This was at least a few years before most of the algorithms that we perceive as dangerous and enshittifying became core to the platform. The specific study they referenced (new link below) argued that there is genetic components in how we perceive our social networks.

https://www.psychologytoday.com/us/blog/psyched/200901/faceb... https://www.pnas.org/doi/10.1073/pnas.0806746106

Where FB and Instagram are to blame is not just being aware of the psychological impact but amplifying it make it worse, especially onto a teen audience that has no capability of distinguishing the real world from social media. To them, it's the exact same. Your online social circle may be all you have in real life, not to mention the cyber bullying, unrealistic body standards and all the other awful parts that come when you gamify and reward capturing people's attention.

I won't deny that individuals are also responsible to guard themselves and especially parents, but these platforms have been accused (and are currently in US court) over the fact that they knew about the addictive potential of their platforms and made no safeguards over improving that. As a platform owner, you are responsible for all aspects of its success and failures, its highs and lows.