They have tens of millions of users and a single large donation won't keep them running forever
and yet the whole Mastodon and Briar ecosystems run on pennies.
Mastodon instances with a couple of hundred users can easily cost $100. It's definitely not running on pennies, just that there are a lot of individual people footing smaller bills.
Most Mastodon instances are also collecting donations to cover server costs…
I was wondering this myself, but apparently they are seeing a huge influx of new users right now. This article explores what might be going on.