Facebook recently created a near-copy of a service called Snapchat, which allows users to send one another pictures that disappear after a few seconds, Nicholas Carlson writes. But Facebook's version, called Poke, quickly declined in popularity, raising questions about the company's approach, Carlson writes. "Maybe Poke's failure actually reveals a flaw in Mark Zuckerberg's long-term vision for Facebook as a platform," he writes.