SAO PAULO (Reuters) - Facebook's popularity in Brazil has exploded over the past year, and the company may have its biggest competitor to thank for it. In its recent initial public offering filing, Facebook said its number of active users in Brazil had nearly tripled in 2011, finally placing it ahead of Google Inc's Orkut service as the No. 1 social network in the country. "I can't think of an example where Facebook has grown so quickly," said Andrew Lipsman, vice president of industry analysis at research firm comScore. "It really just skyrocketed. ...