Donations. For example, the Mastodon instance that I’m in sometimes publishes its financial situation. Most of the time the donations cover the server fees comfortably. But since the amount of users keeps growing and people post more, those donations are sometimes not enough. Only once has it been at a point in which I was worried.

Note that since there are less metrics being systematically hoarded from users, the servers need less power to make similar services work. For example, I bet Reddit’s servers have to account with every little movement I do there. But here, in Lemmy, you can see the source code and notice that there’s very little tracked.

Lemmy in particular is powered by Rust. This makes it more efficient than running a server in, for example, Ruby, which is what Diaspora does. But this is specific to Lemmy and not to the Fediverse, since different Fediverse instances can run their servers using different languages.