Early in today’s session, Best Buy’s stock price is getting kicked around on reports that their third quarter earnings fell by 13%. My co-host Sara Ritz, aka the Market Belle, mentioned on the show this morning how she wasn’t surprises at all about Best Buy’s miss. After all, their stores have essentially turned into a showroom for customers who want to go to the store and look for something, then go home and order it on Amazon.

I know that for years I resisted shopping on the internet. But these days—and especially this holiday season—I’m doing the majority of my shopping online. When it comes to sending gifts to family, friends and clients all over the country, the convenience trumps the impersonality. Anymore, there’s really nothing I wouldn’t consider buying online—including my underwear!

As amazing of a job as Amazon has done of helping to make online shopping the norm—and it is just that: transactions to and from the online giant account for some 40% of shipments that go through FedEx and UPS—Best Buy has failed to keep up with changing customer behavior. It’s not that different from the Blockbuster/Netflix changing-of-the-guard that we saw take place a few years ago.

Not all traditional retailers have been left behind. Take Home Depot, another big-box store that is in many places just a parking lot away from Best Buy. A typical Home Depot shelves about 30,000 products; but if you don’t see what you need in the store, a sales associate will take you to in-store computers and guide you through the online shopping process, where hundreds of thousands more items await. It’s a terrific way of bridging the gap between the brick-and-mortar and the virtual, and it seems to be working: Home Depot’s 2011 third quarter earnings were 17% higher the year prior.

As investors, it’s important that we pay attention to these types of things and tuck them away in the back of our brains. It’s a variation on the old Peter Lynch method—the famous Fidelity Magellan manager who used to browse the local malls to find out what the trends were and where future earnings could come from. This method, adapted for a digital age, can be just as effective as looking at a company’s balance sheet. When combined with due diligence about management and governance, practical information can help paint a complete picture of a company—and help you form an educated opinion about its stock, its sector and the economy in general.