Were November's Black Friday sales numbers the beginning of the end for retailers? Despite a 3.5% increase in foot traffic this year, retail sales fell 1.8% from 2011. Was it more than just retailers like Best Buy (NYSE:BBY) losing customers to "showrooming"?

Actually, it just might mean investors can no longer rely upon some traditional metrics they've long used to track the health of retailers. When more sales are coming from a retailer's online sources than from its bricks-and-mortar stores, the traditional meaning and value of foot traffic, sales per square foot, and even same-store sales will be lost.

Bits and bytes rule
Online retailers delivered a cyber gut-punch to their more physical-presence brethren with sales surging 26% the day after Thanksgiving. Online sales crossed over the $1 billion mark for the first time ever, with 57.3 million Americans visiting e-commerce sites -- an 18% increase from a year ago. Best Buy was actually one of the most visited sites, coming in third behind Amazon.com (NASDAQ:AMZN) and Wal-Mart (NYSE:WMT).

The research analysts at ShopperTrak say shoppers went to the mall more in 2012 over the entire Black Friday weekend than they did in 2011, but this didn't necessarily translate into spending more. The weekend totals saw traffic increase to 594 million store visits, an 8.2% jump, but retail sales increased just 2.7%.

Retailers just might be cannibalizing their own sales. Wal-Mart garners more online shoppers generally than does Apple, Best Buy, or Kohl's, with 23% of smartphone shoppers buying at its online site and 24% of tablet shoppers. According to one survey, 41% of those polled made a purchase on their mobile device after shopping in-store, with 31% saying it was because they got a better price.

Charge it!
That could be because retailers are more promotional this year, trying to attract consumer dollars. The National Retail Federation's Shop.org reports that online stores offering free shipping was the norm this year, but only the beginning of the bargains shoppers could find. Coupons, limited-time only deals, and discounts on specific purchases were also the main means by which to score a great price.

That seems to be in line with data by GroupM Next, which found that simply for a 2.5% discount, 45% of shoppers will leave a store and buy the product instead online. A 5% discount will cause 60% of shoppers to do so, and if the discount is bumped up to 20%, only 13% of shoppers can withstand the lure of the deal.

Let's just forget about it
So investors will need to look at the whole picture before deciding whether a retailer's prospects are rising or falling. Retailers like Sears Holdings (NASDAQ:SHLD) themselves started downplaying some of the metrics years ago when they stopped reporting monthly same-store sales, though in Sears' case that could be because it wanted to marginalize the negative drumbeat of steadily falling sales.

Yet if comps are going to become unreliable as online shopping gains ascendancy, then sales per square foot will be less meaningful itself, though it will force the companies to continue the trend of moving toward a smaller footprint. Best Buy has been opening smaller mobile stores while closing its larger big-box stores, and RadioShack (NYSE:RSHCQ) tried to make the most of a minimalist approach by opening kiosks in Target (NYSE:TGT) stores (something that may be coming to an end because of its poor results -- it's had to write off $25 million in charges due to the program).

E-commerce sales volumes are expected to reach $327 billion by 2016, or 9% of total retail sales, says Forrester Research, up 45% from the $226 billion spent in 2012, or 7% of the total. That itself is up almost 12% from the $202 billion spent last year.

Instead of comps, investors might need to come up with new metrics -- sales per kilobyte? bandwidth per dollar? -- to assess the health of a retailer.

Rich Duprey owns shares of Apple. The Motley Fool owns shares of Apple, Amazon.com, Best Buy, and RadioShack and is short RadioShack. Motley Fool newsletter services recommend Apple and Amazon.com. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.