In the course of the past year, online music sales have started to flatten out, according to Nielsen (via Reuters). Sales and subscriptions at iTunes, Amazon MP3, and other American online stores in the first half of 2010 were roughly similar to the first half of 2009. Compare that to a 13 percent increase the year before, a 28 percent jump in 2008, and it looks like the trend is leveling out.

Researchers aren't sure what's causing the stagnation, but disenchantment with the music on sale, economic difficulties, and confusion over the many sources of where to get it from, are likely to be contributing to the effect. Also, a significant number of desktop listeners are using free streaming services like YouTube as well as mobile phone music services, rather than paying to download tracks or albums.

Still, while stalling US sales are considered unusual, it doesn't mean digital music won't eventually replace CDs. Nielsen pointed to Europe, where digital music sales grew seven percent in the UK, 13 percent in Germany, and 19 percent in France.

Apple still has the largest share of domestic music with 28 percent of all sales, making it mostly responsible for any slowdown. Earlier this month the company launched its Ping social component for iTunes to help its users discover and buy music they otherwise wouldn't.