Facebook may have reported 1.15 billion monthly active users in its second-quarter earnings report Wednesday, but as of Dec. 31, 2012, approximately 5 percent of those may have been duplicate accounts, while 1.3 percent may have been accounts that were improperly classified by users, and 0.9 percent may have been fake accounts, according to the Form 10-Q the company filed with the Securities and Exchange Commission Thursday.

The social network offered more details on how it arrived at those estimates in its Form 10-Q quarterly report:

While these numbers are based on what we believe to be reasonable estimates of our user base for the applicable period of measurement, there are inherent challenges in measuring usage of our products across large online and mobile populations around the world.

For example, there may be individuals who maintain one or more Facebook accounts in violation of our terms of service. We estimate, for example, that “duplicate” accounts (an account that a user maintains in addition to his or her principal account) may have represented approximately 5 percent of our worldwide MAUs as of Dec. 31, 2012.

We also seek to identify “false” accounts, which we divide into two categories: user-misclassified accounts, where users have created personal profiles for a business, organization, or non-human entity such as a pet (such entities are permitted on Facebook using a page, rather than a personal profile, under our terms of service); and undesirable accounts, which represent user profiles that we determine are intended to be used for purposes that violate our terms of service, such as spamming.

As of Dec. 31, 2012, for example, we estimate that user-misclassified accounts may have represented approximately 1.3 percent of our worldwide MAUs, and undesirable accounts may have represented approximately 0.9 percent of our worldwide MAUs.

We believe the percentage of accounts that are duplicate or false is meaningfully lower in developed markets, such as the U.S. or Australia, and higher in developing markets, such as Indonesia and Turkey.

However, these estimates are based on an internal review of a limited sample of accounts, and we apply significant judgment in making this determination, such as identifying names that appear to be fake or other behavior that appears inauthentic to the reviewers. As such, our estimation of duplicate or false accounts may not accurately represent the actual number of such accounts.

We are continually seeking to improve our ability to identify duplicate or false accounts and to estimate the total number of such accounts, and such estimates may change due to improvements or changes in our methodology.

Readers: Have you ever encountered obvious fake accounts while using Facebook?

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