Billboard announced today (Dec. 2) that it is expanding its suite of chart offerings to include a new chart, the Social 50, presented by ZYNC from American Express, which ranks the most active artists on the world's leading social networking sites.

Rihanna becomes the 'only girl in the world' to top the Social 50 chart in its debut week, followed by Justin Bieber, Eminem, Lady Gaga and Nicki Minaj, respectively, to round out the top five.

The full chart is available at Billboard.com.

Serving as a complement to Billboard's existing weekly rankings, the Social 50 chart will tally artists' popularity using a formula blending weekly additions of friends/fans/followers along with weekly artist page views and weekly song plays on MySpace, YouTube, Facebook, Twitter and iLike. Data from these sites is gathered and provided to Billboard by social music tracking service Next Big Sound.

The Social 50 emphasizes friends'/fans' behavior above other activities, followed by artist page views and song plays. The final ranking also includes a unique metric measuring the ratio of page views to fans.

"The Social 50 provides a weekly snapshot of the artists that music fans engage with the most in the social arena, which in today's world is a significant validation of their investment in an act," says Billboard chart director, Silvio Pietroluongo.

"We continue to adapt the way we chart the changing landscape of music," says Billboard editorial director, Bill Werde. "The Social 50 is yet another step in the evolution of Billboard and an important response to our changing times."

"The volume of online activity that top artists now receive is too big to ignore," says Next Big Sound CEO, Alex White. "We are thrilled that social music activity now receives the same emphasis as the music charts for which Billboard is synonymous."

The Social 50 will appear alongside other popular charts on Billboard.com and Billboard.biz every Thursday, and in Billboard magazine every week.