If done right, a federated network like Mastodon would be much more reliable and cheap to run than a centralized one: when the scale is big, the network has to be federated under the hood anyways (i.e. you have to have several caches in different locations, several database nodes that copy data from one to the other, several moderation teams etc.) and so the network that is federated by design has many advantages over the one that pretends to be centralized.