Chantilly, Va.—Advertising expenditures on social media are expected to enjoy a 31.6% annual growth rate through 2015, hitting $8.3 billion, according to a report from media research company BIA/Kelsey. According to the company's “U.S. Local Media Annual Forecast (2010-2015),” the increase represents a quadrupling of social advertising budgets from the $2.1 billion spent on social media ad units in 2010. The company also forecast that social media's share of all digital advertising expenditures will hit 21.9% by 2015. BIA/Kelsey said that display ads command the bulk of current social media ads, but such text-based ads as Twitter's Promoted Tweets and Promoted Trends units also are expected to increase in popularity.