Well, I usually reply to feedback whether I get it in an e-mail, here at the boards, or through the fanfiction.net review system (I kinda have to have that, 'cause LnC isn't my only fandom).
But every once in a while, there's a piece of feedback that I just don't know HOW to reply to, or if I did reply, it'd just be a minimum response, like, "Thanks, I'm glad you liked it!"
Don't get me wrong, I've sent the minimum response before, but I really prefer to have something else to answer--like if they mention a certain part of the chapter/story, then I might like to elaborate on my thoughts from that part.
Or if they encourage me to keep writing a WIP, I just HAVE to reassure them that I WILL be continuing it--or let them know that I won't be continuing others.
So, even though I'm most likely to answer a feedback e-mail, if there really doesn't seem like anything to answer, I MIGHT not answer it.
Then again, it does feel kind of great just to say, "Thanks, I'm glad you liked it!"
