Digital music may be closing in on its ceiling. A new study by Billboard shows that more people than ever are buying music on the internet, but that growth has slowed. From 2006 to 2008, the sales of digital music tracks grew by between 226 and 228 million, but in 2009 that growth plummeted to 90 million. A turn that sharp suggests that digital music sales may have already attracted the majority of consumers who will buy online, and that future growth will continue at a gradually decreasing rate. Check out more here.