Facebook's latest figures showing growth in global users also suggest as many as 83 million may come from dubious sources - duplicate accounts, pages for pets and those designed to send spam.
Facebook members grew to 955 million at the end of the second quarter, but some 8.7 percent may be dodgy, the company said in its quarterly filing with the US Securities and Exchange Commission.
There are "inherent challenges" in measuring usage "despite our efforts to detect and suppress such behaviour," the social network said.
It said duplicate accounts - when a same user maintains more than one account - may represent some 4.8 percent of active users.
Another 2.4 percent may be for a business, group or "non-human entity such as a pet" and 1.5 percent are likely "undesirable" accounts that use the accounts for spam or other malicious activity.
"We believe the percentage of accounts that are duplicate or false is meaningfully lower in developed markets such as the United States or Australia and higher in developing markets such as Indonesia and Turkey," Facebook said in its filing.
"We are continually seeking to improve our ability to identify duplicate or false accounts and estimate the total number of such accounts, and such estimates may be affected by improvements or changes in our methodology."
The number of real users is critical for Facebook as it seeks to secure advertising revenues from the world's biggest social network. Some analysts have expressed doubts that the company can boost revenues.
Graham Cluley of the British security firm Sophos said fake accounts are a fact of life but could affect Facebook's bottom line.
"Clearly all Facebook users are interested in the site becoming a safer place, and the level of spam and malicious links being minimised," Cluley said in a blog post.
"But more than that, companies who are considering advertising on the social network want to be sure that any 'likes' they receive are from genuine users, not bogus accounts."
Facebook shares closed down more than four percent at $20.04, a sharp 47 percent decrease from May's offering price of $38.