Facebook has "likes", Instagram has "hearts", Twitter has "favorites" & "re-tweets", Pintrest has "pins", reddit has "upvotes". Each of these methods of showing you enjoy something have different levels of motivation to get people to actually do them and have different levels of pleasure when you see that other people have done them to your media.
Using a simple photo sharing app as an example and considering all of these methods that already exist and expanded on with ideas of your own, what form of showing you enjoy something fosters the closest community with users wanting to be the most active?