It's extremely hard, to convert free users to paid, when you have not attached them tightly to needle of your service. My friend tried to do similar thing in h

It's extremely hard, to convert free users to paid, when you have not attached them tightly to needle of your service. My friend tried to do similar thing in his startup (very much like Disqus), but he have not success in converting small sites to paid services, so concentrated on totally different market - large corporations, like FT, Washington Post, etc.