Facebook has admitted that as many as 83 million of its users could be fake or inactive accounts, equivalent to almost one in ten of the site’s members.

Buried in the company’s lengthy filing to the US Securities and Exchange Commission (SEC), Facebook said that it had swollen its user numbers to 955 million, but of these 4.8 percent were believed to be duplicate accounts, 2.4 percent were “user-misclassified” accounts (joke accounts or pets), and 1.5 percent “undesirables" (spammers).

On this basis, 8.7 percent of Facebook’s membership is of no use to advertisers, equivalent to 83.09 million accounts.

Even then, the company said it was not absolutely certain about these numbers as it based them on a sample of live accounts rather than an accurate classification; the number of suspect accounts could be lower or higher.

“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,” Facebook promises in the filing.

In June an analysis by Capstone Investments suggested that Facebook's user base in developed countries such as the US was falling despite growth elsewhere. This drop – and the fact that a chunk of its membership aren’t even real accounts – wouldn’t have mattered if the company didn’t have shareholders to worry about.

Some of the investors will take the latest news news as further evidence that the site's popularity in its heartland has peaked for now.