Facebook's latest figures showing growth in global users also highlights concerns that some may come from dubious sources - duplicate accounts, pages for pets and those designed for spam.
Facebook members grew to 955 million at the end of the second quarter, but as many as 83 million 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 per cent of active users.
Another 2.4 per cent may be for a business, group or "non-human entity such as a pet" and 1.5 per cent 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 get advertising revenues from the world's biggest social network. Some analysts have expressed doubts that the company can boost revenues.