Lemmy is a decentralized, FOSS platform built by a Communist explicitly as an answer to Reddit. The people on Lemmy trend leftward, obviously, but that’s because the very foundation is a rejection of Capitalism. If you want Capitalist Lemmy, there’s Reddit.
FOSS itself is leftist, and a rejection of Capitalism. The ability for the users to simply fork off if they don’t like the way something is heading is precisely an advantage of leftist organization, which is impossible with Capitalist Reddit.
Truth Social and Gab are built on Mastadon, yes. FOSS itself is a rejection of Capitalism, Capitalists going in and taking advantage of existing leftist infrastructure doesn’t mean the infrastructure itself is Capitalist.
Your last paragraph is a complete non-sequitor. Much of the USSR was indeed a failure, there was a ludicrous amount of corruption at the Politburo level, and the further up you went the less democratic it was, as only local Soviets were purely democratically accountable to the Workers. With each rung you went up, it was less accountable to the Workers. However, absolutely none of what you say about competition, the USSR, or otherwise follows logically.
Communism itself doesn’t depend on everyone following in lock-step, Capitalism does.