Are these sites a thing (small sites that can subsist on their ad revenue)?
If I couldn't pay the bills for one of my services, I would rather introduce a paid tier, or at least a paid ad-free version. Or I would ask for donations whenever the money runs out, like Wikipedia does.
A paid tier is a surprisingly narrow business model. People aren't willing to pay for much, and not every website has such an explicit product. A lot of that is just conditioning, as people aren't used to having to pay for anything since ads. And most people see ads as zero-cost. The second you start charging a dollar, now you're the guy charging the dollar that nobody else is.
How many forums do you pay for?
And donations are almost always a joke. Else you wouldn't have to beg in a way that puts most ads to shame: https://dl.dropboxusercontent.com/spa/quq37nq1583x0lf/nv9ad0..., and that's for a website that brings value to just about everyone that touches the internet.
If I couldn't pay the bills for one of my services, I would rather introduce a paid tier, or at least a paid ad-free version. Or I would ask for donations whenever the money runs out, like Wikipedia does.