Well, it was true decade, maybe two ago. Stuff got faster and therefore cheaper too.

Server written in reasonably fast software on $10 (well, probably some extra bux in storage if community is image-heavy) VPS can reasonably serve tens of thousands of users in typical forum/blog/hn-esque format. Mastodon isn't but I don't doubt someone would rewrite it in something faster if there was a need for it.