commentrarian

A combination of the words "comment" and "contrarian" used to describe a person who frequently posts facebook/twitter comments about their opposing view and who rejects or resists the majority opinion.

Gene disagrees with everything. He's such a commentrarian.

I can always count on Jean to make a sarcastic comment on my wall. She's a hateful commentrarian, especially about religion and politics.

He is a total commentrarian. He will disagree with everything everyone says... all the time.