>You can choose who you share your stories with on Facebook.Sure, I get that from a pure functional perspective. However, I'm talking about psychology of app usage.
Consider an example of an iPhone home screen.[1] The user sees the Snapchat icon and a Facebook icon next to each other. I believe that many people (millions) will mentally compartmentalize the yellow ghost as "my real friends" and the blue "f" as "every random acquaintance that ever sent me a friend request and I never said no because of etiquette."
Usage of each icon evolved to become its own "world" so to speak. Sure, one could theoretically create sub-worlds inside of Facebook but a customized "real friends of Facebook inside of Facebook" seems to be more work than just pushing the button with the yellow ghost. Staying inside the Snapchat world also minimizes accidentally sending the wrong Facebook-story to the wrong Facebook-friend.
[1] http://i.imgur.com/CV91LsE.png