I'd say it depends on the comment or post. If one makes you feel good or smile, then by all means drop them an IM or a thank you in thier journal. You could also check out thier gallery or journal in return as they may be wanting to start a friendship or establish contact. For a more general way to go type post, maybe just saying thanks in the gallery or a quick stop by thier journal for a thanks would work. What ever makes you feel comfortable and you have time to accomplish.