Reply in the same comments that they did, or you could always find a piece of work they did and leave them an equally awesome comment on one of their projects.
I like to send personal messages through our messaging system. To the artist...the commenter or whoever. It's fun, and you will make lots of new friends this way.
I like to thank each person individually within the same comments section. If i'm not around for a while and a lot of people have commented, then rather than thank everyone individually in the same post I'll give a "thanks everyone"! Although I do acknowledge ALL hearts every time.