BBC News: In a filing with the SEC, Facebook revealed that it believes that 8.7 percent of its 955 million active users may be fake. Specifically, it said that 4.8 of profiles were likely duplicates of other accounts, 2.4 percent were user-misclassified (accounts for pets, clubs, etc.), and 1.5 percent were "undesirable" accounts used for spam or other nefarious purposes. In all, Facebook believes it has 83.09 million accounts that don't belong to real people.
Because Facebook relies on advertising for revenue, the fake users could impact its business if they cause advertisers to turn away from the social media site.