I believe it is a way to make the reader feel validated also and that we took the time to notice them, kinda like a text back. If someone texts me and tell me they like what I said, I would feel the need to say "thanks", however It's a matter of choice. It's not a right or wrong thing to do.
At times I may not be able to reply right back because of work, family, etc. but when I can I do let others know that I appreciate their comments, but that's just me. :)