Something like Facebook obviously costs a lot more to run, as it stores photos and video and also provides each user with an individually generated feed. In contrast HN is rather lightweight and basically serves mostly static cachable content to all users. I believe it was stated somewhere that the entire HN comfortably runs on a 64GB 4/8 bare metal server. On the extreme side I host my personal static web site on my home DSL connection fronted by Cloudflare, as the CDN does all the work and the bandwidth used is minimal.
To the people running these new communities the software cost is low as they're based off an open-source service of choice (Lemmy, Mastodon, etc.), and as we saw on Reddit moderation can be done by volunteers. Are the remaining costs for admins, hosting, etc. feasible for a non-profit with some user donations? And can we create more efficient platforms with a plainer style that will minimize the server and electricity costs?