I read an article about this subject. May be it can help you.
Investigating MOOCs Through Blog Mining - Yong Chen Old Dominion University, USA - "Google Blog Search (http://www.google.com/blogsearch) is specially designed to retrieve content from blogs that are freely and publicly available on the Internet".
I agree with Gunawardena. I did some experiments using Weka. The summary can be accessed here --> http://pt.slideshare.net/claudiodelima/inteligncia-artificial-inferindo-resultados-e-tendncias