Back in September 2008, Best Buy (NYSE:BBY) agreed to shell out $121 million for Napster. The giant retailer had high-hopes for the deal — after all, the online music service had about 700,000 subscribers as well as a well-developed mobile platform. Add in the heft of Best Buy, and you’d get a boost in growth, right?

Didn’t happen. And this week, Best Buy gave up on things, agreeing to sell Napster — whose service shrunk to between 300,000 and 400,000 users — to Rhapsody. The price tag was not disclosed.

The online music world has changed radically during the past couple years. Some of the main drivers include the dominance of Apple’s (NASDAQ:AAPL) iTunes, as well as the hyper growth in online music operators such as Pandora (NYSE:P), RDIO, Mog, Slacker, iHeartRadio and Spotify.

Unfortunately, Best Buy wasn’t nimble enough to deal with the major shifts. And the company’s brick-and-mortar business could have used the help. Same-store sales have been anemic, and

Amazon (NASDAQ:AMZN) continues to take away market share, with Amazon’s newly unveiled Kindle Fire tablet posing yet another threat.

True, it never is easy for a traditional company like Best Buy to move into new waters. Yet some of its rivals have shown they can get traction. Just look at Wal-Mart (NYSE:WMT). The company’s video streaming service, Vudu, has an extensive content library and is seeing strong customer adoption. According to a report from IHS, its market share has gone from 1% to 5.3% over the past year. Wal-Mart purchased Vudu in February 2010.

So shareholders of Best Buy have plenty to be concerned about. There is no clear vision of a digital strategy. And in today’s world, it can be extremely tough to play catch-up, especially when the competition only gets stronger and stronger.

Tom Taulli is the author of “All About Short Selling” and “All About Commodities.” You can also find him at Twitter account @ttaulli. He does not own a position in any of the stocks named here.