Facebook Inc., owner of the world’s largest social-networking site, said its advertisers have more than quadrupled since the start of 2009 as marketers aim to get their products before a growing global audience. Facebook, which has been opening offices around the world to serve ad clients, doubled the number of salespeople last year from 2008, according to an e-mailed statement. The company didn’t disclose the number of advertisers or salespeople. Companies are boosting ad spending after a lull during the recession in a bid to reach consumers who spend long stretches on the Internet. Facebook, which has more than 500 million users, counts Procter & Gamble Co., Toys “R” Us Inc. and Virgin America Inc. among its customers. It’s relying on ads to maintain sales growth and lay the groundwork for a possible initial public offering. Facebook sells ads that are placed on the home pages of users, where the latest comments, pictures and links from their friends are posted. The ads also show up next to profile pages, which may include a person’s information such as gender or birth date.