Social media AND smartphones became popular around that time. I think it's the toxic combination that's the worst - easy, low effort dopamine hits that are available everywhere via your phone, whenever you are bored.

In 2013 social media was still a textual medium, right? There was Vine, but that died pretty quickly, from what I remember.

If social media and smartphones are the problem, I would have expected that results for English proficiency would be steady until the advent of TikTok, right?