LinkedIn became the third social media company in a week to lose as much as a quarter of its valuation in minutes after reporting a weaker outlook, on a steep slide in European advertising as well as the costs of acquiring education site Lynda.com. The business networking site saw some $7bn wiped from its market capitalisation in after-hours trading, as it warned sales in the current quarter’s revenues would be $45m below Wall Street’s forecasts at $670m-$675m, in part because of currency fluctuations.