Social Value is the amount of behavior that one person generates among their friends. An anology might be the ripple on the social pond. Let's say this person goes to see a movie, listens to a song, or plays a game. Now let's say that this person is influential. How much more likely are their friends to go see the movie, hear the song, or play the game?
Each person has a unique amount of influence in their social network, and on each friend. Maybe you are highly influential on Bob, but Steve doesn't care what you do. What the Social Value algorithm does is to add up all of your influence and put it in units we care about--like sessions, time, or dollars.