It’s a testament to the accelerating death of the CD that Apple’s iTunes Store became the third-largest music retailer in the U.S. last quarter:

ITunes had a 9.8 percent market share in the first quarter, ranking behind Wal-Mart Stores Inc.’s 15.8 percent and Best Buy Co.’s 13.8 percent, according to The NPD Group. Online retailer Amazon.com’s share was 6.7 percent, slightly ahead of Target’s 6.6 percent, NPD said.

And questions about how exactly they’re measuring the rankings seem baseless, according to the survey methodology:

The firm counted every 12 tracks purchased online as equivalent to an album in compact disc format, said Russ Crupnick, NPD’s vice president.

NPD’s survey does not include mobile music sales, nor does it factor in revenues.

That’s significant, because it helps de-emphasize the apples-to-oranges comparative between predominant single-song sales on iTunes versus predominant whole-album sales in other outlets. If anything, Apple would come even stronger in a revenue comparison.

You can extrapolate from this how much music-buying has changed over the past decade, which has seen brick-and-mortar music stores become an endangered species. It’s also worth noting how much this convergence has helped hasten the adoption of broadband, the growth of social networking and other developments to the formerly narrow-band Internet. The relationship between music and the Web has been practically symbiotic.