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Facebook is planning another big product launch Thursday — its fourth in six months — in a show of the company's determination to defend its position as the world's leading social network against a host of rival apps and other challengers.

"A small team has been working on a big idea," Facebook said in a note delivered to tech reporters. The invitation, first reported by ABC News, didn't elaborate but added: "Join us for coffee and learn about a new product."

The announcement fueled speculation that Facebook may be working on a service to draw updates from blogs and other websites, since rival Google is planning to end its popular Google Reader service at the end of this month. Facebook, however, declined to say more about the event.

The company has hosted three other high-profile events this year to unveil new features that CEO Mark Zuckerberg described as major advances for the social network. Those earlier announcements involved a sophisticated search function, an overhaul of Facebook's News Feed and a new smartphone interface for Android devices.

Analysts say it's important for Facebook to introduce new products in the face of constant challenges from rival Internet companies and social-networking startups that appeal to younger Internet users as new or trendy alternatives. Facebook claims more than 1.1 billion active users around the globe, but its once-phenomenal growth rate has slowed in the United States.

Facebook's new Android phone interface proved unpopular with many users, save those who are the heaviest users of the social network. But Zuckerberg has downplayed the early reaction and said the company will keep adding features and improving its mobile service.