I assume that the precedent is set with “letters to the editor” in a public domain.
If no login is required, the subject matter is public.

However, there is difference.
1. With a newspaper, there is no easy way of seeking consent. With a blog or twitter or facebook…there is. Post a comment and ask does not seem too arduous.

With the mooc, the use for research is an interesting one as the course advertises this on one site (its own,and in amongst a huge amount of writing that might not be read), but the data collected is on another site- the writers own, where the writer may never have read of intent for use for research…now that just has to be dodgy. So having read the Aoir and informed by Buchanan, E. A. (2004). Readings in virtual research ethics. issues and controversies. Hershey: Information Science Publishing.

how about doing no harm as primary…
ask, its not like its something likely to be made worse by asking…

As always, never post anything on the net that one isn’t prepared to shout in a full lecture theatre.