User scaling seems easy enough.

Its "community" (on Lemmy), or "magazine" (on kbin) scaling that seems hard.

But since each server has a local-copy of the community that its serving out, maybe the hardest part has already been solved by the Federation model. Each federated-instance is effectively a proxy / front-end for the users on that instance.

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I guess Mastodon is way larger than Lemmy though and they haven't had issues yet.