Facebook Inc.'s initial public offering has so far generatedlower-than-expected demand from institutional investors who areconcerned about the company's growth prospects, people withknowledge of the matter said.

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

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