NORTH America has a population of about 500 million, and two-fifths of them are on Facebook. In Africa, with more than 1 billion people, just 120 million use the social network. That’s an opportunity Facebook can’t ignore, though the region poses challenges unlike those the company has faced in more developed markets. To spur growth on the continent, Facebook next month is opening an office in Johannesburg, which will be headed by Nunu Ntshingila, 51, the chairwoman of WPP’s Ogilvy & Mather agency in South Africa. – Bloomberg