This is standard procedure for such a transaction under the Hart-Scott-Rodino Antitrust Improvements Act of 1976.
The transaction, which was announced July 28, 2014, is subject to approval of both companies' stockholders and satisfaction of other customary closing conditions, including expiration or termination of the waiting period under the Act.
The effect of the second request is to extend the waiting period imposed by the Act until 30 days after Zillow and Trulia have substantially complied with the second request, unless that period is extended voluntarily by the parties or terminated sooner by the FTC.
The companies are gathering information to respond promptly to the second request and are cooperating with the FTC in connection with its review. Both companies expect the transaction to close in 2015.