I reply when applicable, like to questions or to clarify, and if someone says something funny I'll reply with a 'xD' or 'LOOOL' if they do get a reaction out of me. Other comments though I will leave alone or simply upvote. Like how do you reply to a basic 'LOL'?
Another thing I enjoy doing is replying in a character's voice.
for example
comment: Gee, Gavin's so handsome!
my reply: Gavin: Oh! Thank you! 
It's fun and ups the level of interaction.