Do you say Thank You?

Maybe they just don't think of it. I find this irksome.
I always acknowledge response and try respond to question asap...

Always, I consider it rude not to.

Maybe sometimes they didn't receive it in the first place. Or they didn't want to bother you. Or maybe they didn't like your advice. Or maybe they are just jerks.

I have replied to questions sent via email sometimes.
I do not expect a thank-you.
I AM, however, in person a cold, cruel, indifferent to the plight of humanity.. nasty bitch... and thus wouldn't worry about being polite.

I agree, it's inconsiderate. I always send a thankyou reply, and as far as I recall have received thanks from all of those who have contacted me.

I find a similar phenomena in forum threads, though not often. A person will post a new thread to which there are responses, but the original poster doesn't acknowledge the responses with a thankyou, what they did with the info, and if it worked. Threads are more useful to the whole community if there is closure.