Mastodon

The more users spread out into smaller, more easily censored instances, the more the remaining fragmented bits of the Lemmy ecosystem still talking to each other will turn into echo chambers full of groupthink. This low threshold for defederation is the Fediverse's greatest weakness. Sure, it's possible to work around it—but how many separate Lemmy accounts are users expected to create? Even if you have accounts on every instance of note you'd need to manually cross-post messages to each balkanized server and their comment sections wouldn't be shared—exactly the sort of thing federation was meant to avoid.

Email, another federated system, has this same weakness. It's why it's increasingly difficult to run your own (outgoing) email server which other systems will accept messages from without going through a well-known third party like Google. Especially when trying to push content to a large audience (e.g. mailing lists), which happens to be Lemmy's core function.