From the Reuters article R&B sales slide alarms music biz we learn

With the exception of new age, the smallest genre tracked by Nielsen SoundScan, R&B and rap suffered the biggest declines in 2006 of all styles of music. R&B, with album scans of 117 million units, was down 18.4% from 2005, while the rap subgenre's 59.5 million scans were down 20.7%. Total U.S. album sales fell 4.9% to 588.2 million units. Since 2000, total album sales have slid 25%, but R&B is down 41.4% and rap down 44.4%. In 2000, R&B accounted for 25.4% of total album sales, and rap 13.6%. In 2006, their respective shares fell to nearly 20% and 10%.
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Merchants point to large second-week declines in new albums. For example, Jay-Z's 2006 "Kingdom Come" album debuted with 680,000 units in its first week and then dropped nearly 80%, to almost 140,000 units.
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"Downloading and Internet file sharing is a problem and the labels are really late in fixing it," Czar Entertainment CEO and manager of the Game Jimmy Rosemond says. "With an artist like Game, his album leaked before it came out, and I had 4 million people downloading it."
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In 2006, the best-selling rap album was T.I.'s "King," which sold 1.6 million copies, while the best-selling R&B album was Beyonce's "B'Day," which moved 1.8 million units. But those are exceptions.
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A senior executive at one major label says ringtone revenue now exceeds track download revenue. And since Nielsen RingScan started tracking master ringtones in September, rap and R&B have comprised 87% of scans generated by the top 10 sellers.

Interscope's Marshall points out that Jibbs, for example, "has sold an incredible 1.4 million ringtones" -- a figure that might well offset lost album revenue. The rapper has moved 196,000 units of his "Jibbs Feat. Jibbs" album since its October 24 release. But figuring the ringtones he's sold at $2 apiece translates into $2.8 million in revenue, the equivalent of another 233,000 albums at a wholesale cost of $12 per unit.

And, Marshall adds, Chamillionaire has moved more than 3 million ringtones on top of scanning nearly 900,000 units of his "Sound of Revenge" album.

Some look at the above data and see it is an argument that the long tail spells the end of the hit. Others look at it and see it as more evidence that piracy is destroying the music industry. Or it may just be a sign that hip hop is finally played out. Me, I look at the ringtone industry and wonder whether it doesn't stand out as an example of where walled gardens and closed platforms have worked out quite well for the platform vendors and their partners yet [almost] detrimentally for consumers.