I don't think it's quite right to call this a "51% attack". That applies more to cryptocurrency networks, or networks built on consensus.

If a single Lemmy instance gained 51% market share, they'd have almost as much to lose as the rest of the fediverse by defederating. There's really no magic number here.

It's also based more on communities than on users. It's easy for a user to move. It's very difficult for a whole community to move.

I think the most important thing in the fediverse is to actively encourage communities to distribute themselves somewhat evenly across instances. It won't matter if Meta has 90% of users if those users are largely interacting with federated communities. Then if Meta pulled the plug, those users would be motivated to pick up and move to an instance that didn't block their favorite communities.

It's great, for example, that we have specialized instances like startrek.website. Ideally, everyone on Lemmy should be "invested" in multiple instances, to the point that federation is essential for the service to remain useful to them on an individual level.

I think the fediverse needs something akin to antitrust laws that encourage and perhaps even enforce competition. I'm not sure how this could actually work though.

Already, I think the largest Lemmy instances are big enough to threaten the health of the network. Shop local and support small instances!