Google has amassed 150,000 advertisers worldwide, thanks to an aggressive international growth strategy in Web search that now includes Spain, the company said Thursday. The Mountain View, Calif.-based search company said that along with its new country-specific site in Spain, Google.es, it also opened an advertising-sales office in Madrid to cater to marketers in that region. The company now operates in 21 countries. Google said that it has signed 150,000 active monthly advertisers in part because of its growing good name in Web search around the world and in part because of the attractive return on investment it offers advertisers. It lets advertisers bid for placement atop or next to search results related to keywords. Those ads perform as much as five times better than traditional banner Web ads do. "Google has been interested from day one in trying to serve the world's information to users. We're now starting to serve the world's advertisers," said Tim Armstrong, vice president of advertising at Google. News.com has more.