In contrast to their other engineered addictions my hunch is (with the caveat that I am not remotely a psychologist) that an important mechanism that enabled this regression was essentially incidental to the nature of these platforms:
Normal people for the first time got a public podium (or they think they do) and thus behave as if the whole world is watching.
Positions and arguments that earlier on were discussed in smaller and more forgiving circles are now in the open.
People get trapped, have to defend a public record, they feel exposed, must double down.
They seek comfort in similarly thinking corners, in newly formed clans, which are, as it happens, much easier to find online and encouraged by the algorithms of the platforms.
The result is large numbers of people absorbed into toxic, self-generated and self-excited virtual realities. A social mechanism that was always active at a low level, but is now on steroids and out of control.
This is just one of the traps: from selfies to influencers, from trolls to click farms the list goes on and on.
The positive impact of "connecting people" is drowned in wave after wave of negative phenomena. If we ever get out of this era intact we'll think of it as a nightmare.