There are not many people in the world qualified to give Mark Zuckerberg advice at a time like this. But perhaps one of them is Bill Gates.

Facebook’s boss has a close friendship with the Microsoft founder, who is three decades his senior and one of Zuckerberg’s heroes. Zuckerberg's formative computing years, in the Nineties, came when Gates was on top of the world and when Microsoft was not only the world’s most powerful technology company, but also the coolest.

There are a lot of similarities between the two: both are Harvard dropouts who left to form the companies that would make them among the richest men in the world. And both have pledged to give away almost all of their wealth to charitable...