What do we call someone who asks a question and never responds to answers/comments?
Think of it more as "a pattern of behavior" (somebody who has asked many questions and never responded).
I had unresponsive in mind, but that doesn't say that they asked in the first place.
Best Answer
Perhaps drive-by asker or drive-by participant, to convey that he comes in, makes his post/comment/etc, and then continues on his merry way. Related, from @onomatomaniak in comments: ask and run.
In other contexts we call someone who takes but never gives -- for example, communal snacks at work -- a mooch (or moocher) or a parasite. Depending on the specific case I don't see a problem with using those words online. Freeloader also comes to mind, though it is more general.
Edited for question revision: mooch, parasite, and freeloader are all perjorative; the drive-by phrases are probably slightly negative but not as much as those.