Online real estate marketplace Zillow (Z) returned to a profit in the fourth quarter of 2011, as revenue soared on increased mobile usage and website traffic.

Zillow reported earnings of $922,000 in 4Q, or 3 cents a share, meeting analysts' estimates, up from a loss of $486,000, or 4 cents a share in the year-ago period.

Revenue rose 108% to $19.9 million from $9.6 million in the last quarter of 2010.

In a quarterly conference call Spencer Rascoff, CEO of Zillow, said he was surprised by the amount of display revenue because it is less predictable than Marketplace revenue, since the marketplace requires a long term contract and display advertisements do not.

While the revenue from display ads was surprising, he said he would like to see a lot more. He said agents typically spend 10% of their income advertising their services, and Zillow receives – on average – less than 1% of that – even given the huge profit margins posted for 2011. He called the statistic “embarrassing.” 

For the full year, Zillow swung to a profit of $1.1 million, but did not record a per earnings share, up from a loss of $6.8 million, or a 53-cent-a-share loss in 2011. Full year revenue rose 117% to $66.1 million, up from $30.5 million in 2010

The company began trading publicly in July.

The fourth quarter was a busy one for Zillow, which expanded its subscription-based Premier Agent advertising program in October, integrated the Zillow Mortgage Marketplace into the Zillow Real Estate iPhone app, and launched the Zillow Mortgage Marketplace on AOL in December.

"Our continued product innovation, particularly on mobile and in expanding the types of services we offer real estate professionals, is fueling our growth and expanding Zillow’s total addressable market,” said Rascoff in a statement. “Additionally, our revenue model spans across the Web and mobile, which positions us exceptionally well for further revenue gains from mobile in 2012 and beyond.”   

In December 2011, homes were viewed on Zillow Mobile nearly 100 million times, or 36 homes per second, the company said.

Average monthly unique users grew 86% to 23.5 million in the fourth quarter of 2011 compared to 12.7 million average monthly unique users for the same period in 2010.

Zillow said said last month was a record month for traffic with 31.7 million unique users visiting the site, representing a 102% growth from January 2011.