Here is your guide to better post commenting...
Like - To be used to signify that you like the content of the post or the way it fits into the current thread discussion (can be intechanged with 'agree')
Thanks - To be used to actually thank someone for their comment...primarily if they have given you a compliment or provided assistance on a subject that helps you
Agree - To be used when you agree with a post in it's content, perspective or have a similar perspective yourself (can be intechanged with 'like')
Disagree - To be used when you do not agree or do not like the content of a post...or just want to poke at the author of the post or make them wonder why you disagree
Either that or you can just randomly hit whatever button you feel like at the moment...or whatever your fat finger hits.