To more directly answer your question, though, you could also look at doing "pro" level subscription memberships if that applies. That can work well.
If you can add some extra/useful features to the site, why not setup a premium subscription? Set the price low to start and that will likely get more revenue this way.
Good luck!
With 2k users, there'd better be.
I'm no expert, but a donation link would probably be more honest, more appreciated, and more effective. Just tell your story. People may surprise you.
If your userbase is such that it may react negatively, and you still need the money, consider a donation or subscription option. Or sell part of the venture, if the eventual upside can justify the trouble for you and risk for investors.