Zuckerberg, the chief executive of Facebook, said that the project has been in development at the firm's 40-strong engineering centre in Seattle, rather than its California headquarters, reports Reuters.
He declined to give any further information, but speculation in recent weeks has suggested that various mobile products are being developed by Facebook, including a long-rumoured Facebook app for Apple's iPad and a specialised photo-sharing service for the iPhone.
Facebook's Seattle office previously played a central role in the development of the social networking giant's new unified mobile site, unveiled in March. A Facebook spokesman declined to comment on the reports.
Yesterday, News Corporation sold MySpace for around $30-$40 million (£18.5m-£25m), a massive loss on the $580m it paid for the social networking site at the height of its popularity, before it lost ground to the likes of Facebook and Twitter.
Zuckerberg's big new announcement also follows Google's unveiling of its own social network this week. Google+ is currently being trialled by a small group of invited testers, but the service is expected to be a key rival to Facebook when it launches.
Last week, Netflix founder Reed Hastings joined the board of directors at Facebook, bringing his digital entertainment experience to the firm.