Maybe Americans are spending more time looking at the depressed values of their homes. Or, maybe residential real estate prices have bottomed out, sparking a new wave of buying. Whatever is happening, Seattle online real estate startup Zillow.com is attracting record numbers.

Zillow attracted 7.5 million unique visitors last month, up 30 percent over December and an increase of 61 percent over January 2008, according to internal numbers released by the company today.

Zillow, which laid off 40 staffers last October, has been attracting a lot of buzz in recent days after releasing home valuation information for various markets across the country. Major news Web sites such as Los Angeles Times and Dallas Morning News have picked up the Zillow data, fueling more traffic to the Web site.

But what is more interesting is the battle between Zillow and Trulia. Zillow appears to be pulling ahead of its San Francisco rival, according to numbers from Compete.com. (Note: Zillow representatives say that Compete under reports its traffic and therefore releases its own internal numbers.)

On a month to month comparison, both sites grew by 18 percent in terms of unique visitors in December. Compete.com puts Zillow's visitor count at 3.1 million and Trulia's at 1.9 million

But Zillow's growth for the year -- up 75 percent -- far outpaced Trulia's growth of 36 percent. And these numbers don't reflect the big January that Zillow just reported.

As recently as last April, Trulia was coming very close to surpassing Zillow. But the gap is now widening, according to Compete.

I heard from a real estate source this week that Zillow has figured out some new search engine optimization techniques, which are helping to build traffic. It will be interesting to see if the heavily-funded company can turn that traffic into revenue and profits.