Facebook Inc.'s initial public offering has so far generated lower-than-expected demand from institutional investors who are concerned about the company's growth prospects, people with knowledge of the matter said.

Some investors expressed reluctance after Facebook said on May 9 that advertising growth hasn't kept pace with the increase in users, said the people, who asked not to be identified because the process is private. Facebook is also telling analysts that sales may not meet their most optimistic projections, two people said.

Facebook executives have another week to market the IPO, set to price May 17, and underwriters are stepping up efforts to drum up interest from large shareholders, one person said. The top end of the price range values the world's most popular social network at $96 billion, or more than Standard & Poor's 500 Index members including Walt Disney Co. and Visa Inc.