I will often comment if I agree with someone particularly and an upvote just doesn't seem enough. Something like "very well said" or "I particularly agree with your second point", it just seems the propper manner for discourse. And doing so you will more rapidly realise when you frequently are agreeing with the same people, oppening avenues of further communication.
Some people may want to remain anonymous in that situation, so if you want them to know who did it, tell them you did it. It would make me smile to get comments like that from someone I wasn't necessarily even talking to. :)