Silicon Valley’s peculiar brand of normal last week generated a lively argument: Did the founders of Snapchat, the mobile messaging app whose disappearing posts make it popular with teens, blow it by turning down a $3 billion offer from Facebook? As if it’s possible for someone in their mid-twenties to blow it when the decision path in front of them is: A) Cashing out at that valuation; B) Waiting for a higher offer; or C) Going public...