Real estate search engine Zillow has identified a big opportunity in the rental market, and was willing to pay $40 million in RentJuice to kick start its plans.

Historically Zillow focused more on mortgages and homes for sale, but as rentals started taking up a bigger and bigger chunk of the pie, the company decided to adapt.  RentJuice, founded in 2009, currently lists one million units essentially that mark its parent company’s first big entry into this space.

“Zillow CEO Spencer Rascoff argues that he sees “a huge market opportunity in rentals. More than seventy percent of movers each year are renters and rental units turn over six times as frequently as homes for sale, yet the professional market is highly fragmented.”

The acquisition of the Irvine, CA-based firm has the potential of putting Zillow in a very unique position, and that is an opening that it most definitely needs to exploit: the competition with Trulia is more intense than ever.

Just a couple of days ago the rivaling marketplace operator introduced a free Android app for that features a few very notable social elements. It allows agents to post check-ins on their Trulia profiles, and includes a number of notepad-like capabilities that make it easier to write down lead-related information such as contact information on the field. This is coupled with a notification system that forwards client messages to the user’s mobile device.

An iPhone version has been available since late last year.

While Trulia’s engineers are working on new solutions, its management is probably more worried about the company’s ever-nearing IPO. Zillow is already a publicly traded company, but to match the success it saw Trulia will probably have to do the same as a few other companies and put its public offering on hold until the anxiety caused by Facebook’s decline wears off.