When someone comments on a Facebook post such as a wallpost, status update, or anything else in an attempt to bring in another conversation and divert attention away from the post and toward themselves.
Jason posted on Marie's Wall:
"Hey...how have you been?"
Jack commented on Jason's wall post:
"JASON! Why didn't you text me back?"
And Jason and Jack have a conversation on the wall post that was meant to be for Marie, and thus Jack postjacked the conversation. Postjacking can also occur in status updates, but it most common on wall posts.