Presumably you know your poster well enough, whether they check on their previous posts (add additional comments or responses)? Or, perhaps you are checking to see if/when they're online and/or what they're doing/reading (if they allow that in their profile) and you hope not too many people will thank them for the same post (as then they might notice "someone" has left but not exactly who).

Just because I "act" nice and play by the rules, doesn't mean I think nice all the time