I can see what you are trying to do but what's the point? Do you drop someone's website because they only made 29 posts one month? Do they get a bill if they don't make 30?

I personally would avoid this type of 'incentive'. Some people might like the idea but I would think it would be abused to the nth degree ~ 30 'me too' posts for a month's free hosting, plus, how do you verify the identity of the person? That would require at least a credit card and who's going to give over a credit card for free hosting?

I think you are setting yourself up for a drop. But who knows? It might work great.