A person who comments on a thread dedicated to a topic they pretend to abhor but secretly find fascinating, if for no other reason than they are compelled to act out a deep-seated impulse to be a total opinionateddouche bag. This condition is often coupled with bitchitus and in some people can lead to chronic dickiness.
If one encounters a threadbear squatting on a site, it is best to let it shit then move onwithout interruption.
Much like the homewrecker, a threadwrecker interferes with the natural flow and order of things, e.g. a thread of comments related to a post on Facebook.
That jerkwad is such a threadwrecker on Facebook!!
Post: JANE DOE is deciding between pizza and mexican food for dinner.
NJ: Oh, I would def have pizza!
ST: Have you tried Jose's Cantina? Awesome?
JW: Hey Jane, is your grandmother out of the hospital yet or did she die already?