Picture-sharing service Instagram (driven only by the popularity of its free iOS app) has hit a whopping two million users already. As TechCrunch notes, the service hit its first million users in just three months, and six weeks after that, has doubled that number. That's pretty astounding growth -- even other popular sharing services usually don't hit those milestones until a year or more after they start operation.
But Instagram certainly seems like it's hitting at the right place and time -- it's a super simple s
