> Display an income bar. If it drops to zero the servers turn off. If it drops below 1 nobody can post. If it is above 1 you have Direct messaging, above 2 you have more features, etc. Keep the communication clear as to what is being provided and how it is being paid for.
This is a fairly normal way to run old-style forum hosting - I remember forums that would display a bar for "this month's hosting costs" or a "hosting costs are paid until [date], donate now!"
> Without an explicitly capped profit, I can't see how this doesn't eventually lead to exploitation of the users.
I don't see what difference capped profit would make. Exploitation of users doesn't usually happen during the starry-eyed "this is going to be a billion-dollar company" stage, it happens in the "is there anything we can do to keep the lights on for another few months and maybe turn it around" stage.
IMO the problem isn't investment per se, it's debt, in a broad sense: spending money now that you're expected to repay in the future, and then struggling to repay it. There are bootstraped, sustainable organisations operating in this area similar to what you're asking for, e.g. Dreamwidth. But those are never going to be able to "blitzscale" or market themselves to the same extent; marketing almost by definition involves spending money now that you hope to recoup in the future, at which point you've already sown the seeds of your ruin if that future revenue doesn't materialize.