I don't think you did anything wrong here. It is a cool little bonus prize you folks get to play with and rewarding folks is always fun. Worst case scenario though is how once this is well-known, some people might seek out the posts rewarding comments and ignore the rest of the posts. If they know about the bonus but don't know where it is, they'll be everywhere, and of course only the quality comments actually get the rewards, so it should come naturally.
It's been somewhat quiet around here lately so these incentives are a good thing. As someone who doesn't use the service, I'm almost jealous!
Almost.