Right. See this post in another thread. I had joined one of the instances mentioned there (mstdn.party, but only because someone I knew went there first when I was thinking of joining in November and was trying to decide on a server to join), and luckily I noticed a mention a couple of weeks ago that the admin was not responsive since late-December and moved to a new instance. But, yes, so far both instance I joined have some sort of voluntary funding (Patreon, Ko-Fi, etc.) to help defray the costs.
But because of federation, it’s unlikely that there will be any one instance that grows large. One of the most popular, mastodon.social, is essentially closed at this point to new members (I think that you can still get an invitation from an existing member.)