Close. The internet was never leftist -- as in opposing capitalism. It was at its best the ACLU wing of liberal, and at its worst the age of consent wing of libertarian.

When social media exploded, the internet condensed to just a few aggregator platforms. You now had all this traffic and attention that could be more easily monetized than a million small websites and forums, and that's what happened. Your few companies that own these aggregator platforms now have an enormous financial interest in (1) keeping content palatable to advertisers and (2) keeping regulation and taxes at bay. They accomplish the second in part by cooperating with the U.S. intelligence community, to the point of becoming one of the many industries with a revolving door between their corporate governance and the parts of the actual government that deal with the industry.

Of course any significant leftist communities on these platforms get snuffed out: big business and the American government hate the left. Your ACLU-type liberals get pushed right or out as the impetus to make money drives every decision, with their free speech language selectively co-opted to protect the right. Then your most right-wing party starts to become openly fascist around the time a fascist buys one of the major platforms and removes even the nominal guardrails against the most egregious fascists.

Now we're here: with a few small non-fascist corners of the internet populated by a mix of leftists and liberals.