R&B star Rihanna has passed Lady Gaga to become the most popular female artist on Facebook.  According to social media statistics tracker Facemount.com, Rihanna is currently sitting at 40,598,000 Facebook fans, moving her just ahead of Gaga, who has 40,565,000 fans on the popular social network.  Gaga still reigns supreme on Twitter, however, with 11.6 million followers, more than any other artist.  The only artist with more Facebook friends than Rihanna, meanwhile, is rapper Eminem, with 43.3 million friends.