How can I predict the post popularity of reddit.com with hidden markov model(HMM)?
I don't think it makes a lot of sense to use HMM's for this problem. What I would suggest is some kind of text-based classifier. 
If you want to use a cool technique, you could use a neural network to learn based on the text of successful posts.

On the other hand, If you want to use an easy technique you could make a predictor for the popularity such as a regression model (try to predict upvotes).