I don't have any figures, let's just 5 servers running containerized environments, with a couple of decent uplinks, it is feasible to handle bandwidth of a million users for a few thousand a month.

With a million users, it's feasible to support the site with donations... no funding or IPO's needed...

In addition, there are numberous open-source frameworks ( like gnu-social etc. ) that enable a single person to deploy the code onto the servers.

So far Mastodon seems to be doing okay on a mix of "I wanna run a server for me and a few friends" and "I'm running a server, here's the Patreon to help pay for it".

"Please Read: A Personal Appeal From <open distributed platform> Founder" a la Wikipedia