Google, Inc. (NASDAQ: GOOG) recently has had a knack for releasing software products that challenge -- and even threaten -- entire industries. Take, for example, last week's enhancement to Google Maps, which caused the stock prices of GPS companies TomTom and Garmin to plummet after Google announced its own GPS service that, in many ways, was a superior product. Yes, it's free with a Google-powered phone.

What's next? How about real estate listings?

Just call it a record year for U.S. homeowners -- a distinction they don't want to repeat.

U.S. homeowners lost an astounding $3.3 trillion in home value in 2008, as the worst slump in the residential real estate segment intensified amid the U.S. recession, Zillow.com announced Tuesday. Further, homeowners lost $1.4 trillion in value in Q4 2008. In 2007, homeowners lost $1.3 trillion in value.

Further, the median home price based on data collected by Zillow.com plummeted 11.6% to $192,119 in 2008. Meanwhile, the percent of homeowners with negative equity -- home values less than their amount owed -- jumped to 17.6% in Q4 2008 from 14.3% in Q3 2008.

The real estate research firm Zillow.com released a sobering statistic, and it took some by surprise: more than one-third of homeowners who bought in the past five years have negative equity in their homes.

Negative equity is owing more on your mortgage than the market value of your home. On the heels of the United States' greatest residential real estate boom in the modern era, how did the above occur?

Two factors, so says economist Peter Dawson.

First, many regions of the U.S., particularly the west, experienced abnormal gains during the 2003-2007 real estate boom. "Total appreciation rates over 300% were not unusual during the period. It was an amazing run, fueled by adequate national GDP growth, and low mortgage rates," Dawson said. "But as we've seen, ultimately the appreciation rates proved to be unsustainable, everywhere they occurred."

Dawson says a 7-9% annual increase in the U.S. median home price is normal, and his models label a 10% annual increase or higher as "outsized" -- a deviation from the mean that calls for a correction at some point in time. "But during the boom, it was not uncommon to see 30%, 40%, 50% annual increases over multiple years," Dawson said. "Clearly unsustainable. Downright frothy. But these conclusions were largely ignored during the boom, on the fallacy of 'what has occurred will continue.' "

Second, a financial habit shifted, Dawson said. Way, way back in the twentieth century, Dawson recalled, the biggest stigma when he grew up in a typical neighborhood in White Plains, N.Y., a suburb about an hour north of New York City, was... Not gaining acceptance at a good college? No. Not getting the hottest date for the high school senior prom? No. "We learned that the Smith's [not their real name] down the street had to take out...a second mortgage," Dawson said.

If you're into residential real estate speculation, you're probably familiar with Zillow.com. Zillow.com gives instant and computerized market value figures and electronic appraisals for almost any residential property in the U.S. No more going to several hundred sources to get all the specific information on a house you own (that may be going up for sale) or for prospective houses that you may want to buy and move into or own for investment purposes.

But, how accurate are Zillow.com's estimates? I've used Zillow.com quite extensively and have found the information to be hit-or-miss. It can be way off and right on target. This, unfortunately, makes it very questionable as a resource in my opinion. For example, the county assessor that values most homes in every state is not exactly precise when it comes to judging the actual market value of a home. I believe this is where Zillow.com gets quite a bit of information. Problem is, the estimate can be 15% to 20% off sometimes -- a huge problem.

While Zillow acknowledges that its estimates can be way off in some cases, it cites that the service it provides is only a "starting point" for people trying to figure out how much a home should cost. This makes sense -- and buyers and sellers should use many more resources for reconciling a home's financial worth in every case anyway.