The "green thumbs" are just a way of telling someone that you like their post. If it is interest, informative, maybe they posted a nice photo or whatever the reason. To give them, just click on the green thumb in the post that you like. When you click on it, it will turn grey show that it has been click. If you click again it will turn back green and remove the thumb the you gave them. The number beside the thumb indicates the number "green thumbs" that post has been given.
