Facebook shares fell to $20 yesterday after having fallen from the opening stock price of $38 a share.

The announcement comes after Facebook admitted that around 83 million of its users were duplicate or illegitimate profiles.

The popularity of mobile devices has limited Facebook’s advertising draw, the company said: "Growth in use of Facebook through our mobile products, where we do not currently display ads, as a substitute for use on personal computers may negatively affect our revenue and financial results," Facebook wrote in its IPO filing in May.