Last week, Google CEO Larry Page met with WhatsApp CEO Jan Koum in a last-ditch effort to prevent the rapidly growing messaging app from selling itself to Facebook, according to three people involved in the deal. The Google chief had learned that a few days before, Facebook CEO Mark Zuckerberg had made a pitch to Mr. Koum to join his company and his board of directors, a bold move Mr. Zuckerberg felt he had to make to preserve Facebook’s dominance in online communication in light of WhatsApp’s growing global popularity. Mr. Page’s message to Mr. Koum: Stay independent as you’ve always planned. You’re a big threat to Facebook. And joining Facebook would have a major impact on “how things play out for years to come,” according to one of the people involved in the talks.