I don't think you should be too worried about it. You've got a pretty loyal (and decently sized) user base. Plus all your existing users have their content tied to your site, so don't go writing any data export features that would even allow them to jump ship.

Its probably a hard pill to swallow, but you said it yourself: they know who you are, they've never approached you, and they are writing their own. Conclusion: for whatever asinine reasons, they've determined consciously that buying OP isn't in their best interests. You courting them isn't going to change that scenario.