Capstone analyst Rory Maher said he used proprietary software to track user numbers by country over the past six months across more than 200 countries and worked out that the social network had shed users in both the United States and Europe. U.S. users declined by 1.1 percent, he said in a research note.

Of the 23 countries where Facebook's penetration exceeded 50 percent, only nine expanded their user base over the past three months, while the remaining 14 countries either had fewer users or saw little change.

Facebook representatives did not immediately respond to requests for comment.

Facebook, which last reported more than 900 million users, has undergone phenomenal growth since its 2004 inception, culminating in a record May initial public offering that valued the company at more than $100 billion.

But the stock has since lost a third of its value. On Tuesday, it slid as low as $27.15, after falling by more than 8 percent on Monday. (Reporting by Edwin Chan; Editing by Richard Chang)

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