LinkedIn Corp, the biggest online professional-networking service, fell the most in almost two years after forecasting sales that trailed analysts' estimates, raising concern that mobile advertising will be slow to kick in.

LinkedIn tumbled 13 percent to $175.59 at the close on May 3 in New York, the biggest decline since Aug 8, 2011. That sliced its year-to-date gain to 53 percent.

Revenue in the current period will be $342 million to $347 million, LinkedIn said this month in a statement. That missed the $359.7 million average analyst projection, according to data compiled by Bloomberg. While the company revamped its smartphone site and announced plans to spend $90 million on mobile newsreader Pulse, ad revenue on wireless devices may not be rising fast enough to keep up with traffic growth, said Aaron Kessler, an analyst at Raymond James & Associates Inc.