Facebook co-founder Dustin Moskowitz is leaving the company to start his own business with Facebook engineer Justin Rosenstein.

Moskowitz helped design the popular social networking site. Now he plans to launch a software startup, according to reports on the Valleywag.com Web site. The Silicon Valley publication said that CEO Mark Zuckerberg notified Facebook employees in an e-mail.

Rosenstein had given high praise to the social networking site when he departed Google in 2007 to join it. He said the company did with 60 engineers what 600 other engineers could not pull off and was on the verge of "changing the world." He spoke highly of the talent there and indicated that one person's contributions could make a difference. Since then, Facebook has hired several former Google executives.

Rosenstein posted a statement on his Facebook profile saying that he and Moskowitz plan to create software that becomes as indispensible as Facebook. He also talked about how Facebook had changed the world, according to an internal e-mail published by Valleywag. He pointed to the ease with which users could look up phone numbers and e-mail addresses and said it is now more difficult for terrorists in the Middle East to isolate impressionistic youth, though he didn't cite details.

The e-mail said that Moskowitz discovered a second passion while building Facebook: "Making companies themselves run better." He went on to stress the importance of improved communication and increased transparency.

Finally, Moskowitz reassured that "Facebook is moving in the right direction."

Zuckerberg indicated that he would continue to seek advice from Moskowitz, who was his roommate in college.