As Facebook approaches another subscriber milestone, its growth seems to be slowing.

The social-networking giant had 687 million users at the start of the month, according to data from Inside Facebook Gold. However, the data shows that Facebook's growth rate has declined for the second straight month.

Until two months ago, the typical growth rate for the previous year was at least 20 million new users, but Facebook logged 13.9 million new users in April and 11.8 million in May, according to Inside Facebook's data. While much of the company's growth comes from late-to-the-party countries, early adopting countries such as the U.S. and Canada posted significant user losses during May, according to Inside Facebook data. The U.S. lost nearly 6 million subscribers to end the month with 149.4 million users, and Canada lost 1.52 million users to finish with 16.6 million.

Facebook representatives did not immediately respond to a request for comment.

The loss of users in early adopting countries may reflect frustration with the company's ongoing effort to allay security and privacy concerns.

In May, Facebook launched several new security features designed to protect users from malware and from getting their accounts hijacked. But then the company raised the hackles on privacy advocates with the quiet launch of a facial-recognition tool that automates photo tagging and suggests friends to tag in photos based on what they look like. The new feature has reportedly attracted the attention of European Union data-protection regulators.

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