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1.
One who disrupts the pace, ryhthm and progress of a Facebook thread by means of their insipid, irrelevant or offensive comments.

A person who by virtue of their lack of wit or perception, inability to grasp sacrcasm or innuendo or refusal to add meaningful or interesting commentary, can single-handedly bring a successfully flowing thread to a brake-squealing halt.
Allison: That was some funny shit that got posted last night!
Rhonda: Yeah, until that threadtard jumped in and sent the whole thing spiraling into the crapper.
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Gina: Nobody puts Baby in a corner.
Doofus: That's from Dirty Dancing!
Allison: No shit, Threadtard!
by Grass-Abber December 11, 2010