Reddit, which I think is the actual ground zero for this spreading online and not Tumblr (unless users of the latter subverted the former or there was significant overlap), spread literal mis info around the middle 2010s even before Trump started his campaign
Yes and no. It's an opinion humanities majors have held for some time given their revolutionary fantasies, but the spread of it is a little more intricate than people think:
-SomethingAwful was full of Bush era smug libs, who even as far back as around 2003 had been infiltrated and was being actively propagandized by government actors if the old accounts of Goonstation are anything to go by, where spergs trying to play vidya are getting lectured on what we now recognize as proto-intersectionalist philosophy.
-Said smug libs go on to commandeer Reddit through subs like ShitRedditSays, where they disrupt the previously libertarian site by coordinating mass report brigades to humiliate and remove people and subs they find undesirable. Many then go on to become sub owners, admins, or jannies themselves .
-Around 2016, ShareBlue pays several million dollars to buy out r/politics alongside many other major subs and flood them with propaganda. The upvote/downvote system previously used to weed out low effort trolling is now weaponized to more adequately push out dissent against the owners of the propaganda mills. The Goons' raid and false flag tactics are tolerated because they suit the whims of the new owners of the major subs.
-In parallel to mid 2000s SomethingAwful, there is Livejournal. In addition to being sort of the ground zero for a lot of cringe fujo shit, also had a surprisingly active community of adult humanities majors. Things like radical queer theory and other closed circuit philosophy gobbeldygook that was once the sole demense of overly read grad students now had a proper internet presence. It was still somewhat niche even there, but hold with me for a second.
-Enter Tumblr, which starts blowing up around 2010. The userbase is largely art and fandom focused teenage girls and "sensitive" guys. Not dissimilar from LiveJournal, which is why a lot of users from there start making the jump. The humanities majors included. Now you have mid 20s and early 30s women with humanities degrees playing queen bitch to teenagers... and the propagandizing starts.
-The reblog system allows for fast information spread and for gigantic pile ons. Now armed with a new ideology in the form of social justice, the base high on fandom and sex politics coalesce into a new and aggressive strain of highly emotional pornbrained activists ready to jump on any dissenters without knowledge or need to read. Tumblr becomes notorious for its userbase of volatile crybullies and children. With time, these teenagers feed back into the university system, already familiar with humanities propaganda.
-This is where things start to merge. In 2018, Apple threatens to pull Tumblr from the App Store because of its inability to keep teenagers from posting nudes of themselves. While sensible at the surface, it's not a problem that is unique to Tumblr, and other sites in its league don't seem to be facing any of these pressures. With apparently no other way to fix the problem, Tumblr bans porn. The previously mentioned lunatics leave the site en masse with quite a few ending up at Reddit.
-With Reddit already under the control of Goons (which also had plenty of Tumblrites among its ranks thanks to the Danganronpa translation fiasco) and DNC shills, the militant progressive activists fit right in. Thanks to the upvote system, their instinct to immediately dogpile and screech at opposition has the effect of actually being able to remove anything that contradicts their worldview.
-As the lines between internet and IRL start to blend, the ability to remove people from comment sections starts extending to the rest of their thinking, and they become convinced that they should be allowed to remove you from reality.
There's some things I didn't mention like the Autism Holy War or radfem groups that did similar things to Goon infiltrators, but that's about the shape of why Reddit turned into how it is now best I can tell.
