One person posts a picture and a friend comments on it, then mutual friends (such as those who are oddly curious about the poster of the picture or someone in it) can take a quick glimpse at that person's life. This is rewarded by the brain with a small shot of dopamine.
This is a function not of the social graph itself (friend) but of the addictive nature of the fleeting glimpses it allows people to have of others.
Suppose you have 500 friends who each have several hundred friends. Some subset of those friends constitutes your "fascination graph". Maybe you think they are hot, maybe you aspire to their lifestyle, maybe you hated them in high school, etc. etc.
In order to continue to succeed, Facebook (and app makers) have to realize that it's not the mundane behaviors of your Facebook friends that are interesting... it's the allure of those on the fascination graph that generates the dopamine and the slot machine-addict kind of clicking that is gold to advertisers.
Facebook has managed to deliver this dopamine in a way that's not creepy (nobody has to go through anyone's garbage, etc.) so the question is, is it ever going to start to seem creepy?