He should buy the New York Times too, and make their articles free to logged in Facebook users. That would bring me back to Facebook more than instagram will (nothing against instagram - it's a great app).
Ha, that was my thought experiment, too. Doesn't seem to be a fit with the current demographic of Facebook, but Facebook is trending towards a "social media conglomerate" to the point where it would almost make sense. Interesting play. (Though back to the point at hand, I doubt you could buy the NYT for $1B, value or not.)