I leave positive feedback to new players who manage to beat me so that they will be encouraged to stay on the site. More new players sticking around means more players who would be ready to buy premium which means more money for Lack to hire extra help with which means more updates with more frequency on CC.
I also leave positive feedback for my teammates after long games. This is for three reasons: first, they have to put up with me bossing them around in the game chat the whole time so leaving them some positive feedback is the least I can do to make up for it; second, they were responsible for the win or/and fun time and sending them positive feedback is my way of saying "Thanks for the win or/and fun time, dude"; third, I don't gain as much points from a team game as I do from the 8-player games that I usually play, so I would like gain a little bit extra, so I go fishing for positive feedback.
Along with that, I leave feedback after speed games as well. Game chat is used more often in speed games than in 24-hour-games, and because of that I get to know the people I'm playing with quite well before the game is over. I tend to lose a lot in speed games, so getting piece of positive feedback is a nice consolation prize. After being sent a positive feedback from someone that I've met and talked to for a while I feel obliged to return the favour.
When I leave feedback I will try to talk about both the player and the game and because of this my feedback is
pretty lengthy.
I voted for the "Yes, I both fish for feedback and leave comments that relate to more general things than a specific game" option.