Learn English – What do you call somebody who asks a question and somebody who answers a question

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.

Petrified is an excellent fit....

2: to make rigid or inert like stone
a : to make lifeless or inactive : deaden
b : to confound with fear, amazement, or awe
_{from m-w.com}