I would love to explore Reddit's influence on this phenomenon. The timelines seem to sync somewhat and it seems like a perfect vehicle for adversarial nation states to use to influence public opinion.
I highly doubt that Reddit per se has any significant effect whatsoever—which is not to say that it can't be an instance of the overarching platforms that cause the phenomenon.
The reason I'm disregarding Reddit as a significant cause is threefold:
a) Reddit is overwhelmingly male, and the most affected group are teenage girls
b) Reddit is not that big of a platform in the teenage demographic
c) Even if it were, its popularity is not in any way homogeneous across countries. If Reddit itself had an effect, the USA would be much more affected by the phenomenon than the UK, for instance.
Also falls perfectly within what Yuri Bezmenov (the KGB defector who explained the Soviet's plan to take down the West through ideological subversion) laid out.
At around that time, you would have the first batch of fully indoctrinated (trained by Socialists from childhood to adulthood, with no oversight or counterbalance) graduating.
That is also the group that sets the stage for the demoralisation phase of the plan and allows for the next phase: destabilisation.
I don't think this is purely a Soviet ploy working out, but you can't help but look at the Western education system and realise that's a big part of the problem.
In that interview he also talked about how the USSR was imminently going to take over the West because everyone had been indoctrinated to communism, so he's not exactly batting 1000. It seems to me he lays out a culture bleakness Rorschach test about "kids these days" that's easy to see ourselves in, and will be able to any point.
