They don't and shouldn't. Communities can't be monetized outside of ads and I've never seen that work long term. It won't make anyone rich or pay back investors either. At most it'll cover server costs.

A common problem is that communities where you can say whatever (after moderation of course) will say things advertisers don't want next to their ads. An extreme example is 4chan, but another one is YouTube. A lot of advertisers pulled out of YouTube because their ads appeared before a video of someone making a racist joke[1].

[1] In YouTube's case it mostly hurt content creators, not Google.