LinkedIn's Marketing Solutions division almost doubled its active advertisers and increased earnings 68% to $83.2 million in the fourth quarter year-over-year, the business social network reports. The growth comes from initiatives with brands such as Citi's Professional Women’s Network and Capital One's Jet: The Business Traveler Network, but the network is currently testing content marketing outside the network with GE, Xerox, The Economist and BlackBerry. "The potentially larger opportunity [than tablets] is smartphones by virtue of the inventory generated there," says CEO Jeff Weiner.