Bloomberg offers a behind-the-scenes look at Facebook’s mobile advertising business today and finds that while it has made big strides, it still has a long way to go. The massive social network has streamlined its ad-buying process and added tools to help advertisers determine their ROI, and it will soon announce a partnership with Nielsen to measure mobile audience sizes. And while the rapid rollout of some ad products have sometimes been “overwhelming to marketers,” Facebook’s strategy of placing ads directly in users’ news feeds is paying off dramatically, with mobile accounting for 14 percent of overall ad sales in the third quarter of 2012 and 23 percent in the fourth.

An important next step for Facebook will be building out its network of local advertisers. The Bloomberg piece notes that while nearly 18 million local businesses use the site, only about 1 million of them are paying customers. I’ve written before that Facebook has done a solid job of not overloading news feeds with ads, which would surely infuriate a substantial number of users. But I constantly see poorly targeted mobile ads on Facebook for products or services I’m completely uninterested in. Growing its network of local advertisers is crucial for Facebook to deliver highly targeted ads to on-the-go users consistently.