Asks a question; never responds to answers/comments

Solution 1:

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.

Solution 2:

I like nonreciprocal.

re·cip·ro·cal Interchanged, given, or owed to each other

Solution 3:

Distracted. It's possible that a person who was interested enough to ask a question, but didn't follow through with the process of discerning a helpful answer, was distracted by something else. It could be a shiny object, or it could be a sick child. It could be one or more of any number of things.

Solution 4:

I don't think there is a single word that captures what you want to say. These terms come to mind:

• unappreciative of the help offered

• uninvolved in the learning process

• exhibits poor etiquette concerning questions asked

• lacks follow-through

• is half-hearted (or apathetic) about his questions

Solution 5:

How about deadbeat? In the same way that a deadbeat dad spawns a human life that he does not concern himself to care for, a deadbeat asker could be construed as a person that spawns a discussion that he neglects to contribute to.