Happy Tildes user here. I believe Deimorz has some stated plans on how it will pay for itself (non-profit, donations, etc) but here's the thing:

I used to ask myself that question, then I realized it doesn't matter. Communities come and go. Slashdot used to be good. Reddit used to be good. That they're not anymore doesn't matter.

It sucks to up and leave a community once in a while, but it's not like you lose everything you did there. Social apps are intrinsically focused on the short term (past & future). It's okay to change which websites you visit once in a while.

I highly recommend Tildes. I enjoy my time there. It's pretty quiet and has very high signal, very low noise. If one day it has to be shut down because the bills can't be paid, that will suck… but there will be others.

Even at this point, the actual bills are already covered several times over, and the site could probably easily grow to at least 100x its current size without needing any more donations. The only real question is whether I continue working on it full-time, but keeping it running isn't in doubt at all, and I can't imagine that changing. There's no investor/owner pressure that would result in it needing to shut down: https://docs.tildes.net/faq#what-if-you-dont-get-enough-dona...

It's much easier when your goal is simply "sustainable" and not "unicorn or bust".

taking out the cost of the workforce (R&D, sales, associated GnA) working on revenue generation, ie. ads, analytics, etc. i'd suppose the rest of the costs of a typical Web2.0 site (like Twitter, FB, Reddit) comes down to basically hosting only and isn't that high - can probably be easily covered by donations. Or a real example - without meaningful monetization and thus related costs, WhatApp was fine with 55 employees serving $19B worth of user engagement.