Facebook’s mobile apps are used by hundreds of millions of people around the world, but they’ve always been both free and ad-free. The latter is set to change in the first quarter of 2012, with Bloomberg reporting that Facebook may introduce its first mobile advertising by the end of March. The story tips the company’s Sponsored Stories ads to make the leap from its website to its mobile apps, suggesting that they will appear in users’ news feeds. TechCrunch has more info, including some quotes from Facebook’s advertising spokesperson Brandon McCormick. “It’s about getting the mobile experience to be really good, then we’ll think about ways to monetise it,” he says. “We want a solution that’s going to work for people, we don’t want it to be interruptive, but we want it to be effective for advertisers.” As our recent With Teeth comment pieces from Darren Hemmings and David Emery show, there’s a lively debate around the effectiveness of Facebook ads for music artists, so the advent of mobile sponsored stories will be followed closely by music marketers.