As if Rihanna and Justin Bieber needed another chart to conquer, Billboard has given them one. The magazine has just launched the Social 50 chart, which ranks the most active artists on the world's leading social-networking sites, and the two pop juggernauts have landed at the top of the tally.

Rihanna and Bieber place first and second on the list, respectively, followed by Eminem, Lady Gaga and Nicki Minaj. Filling out the top 10 are the Black Eyed Peas (#6), Katy Perry (#7), Ke$ha (#8), Linkin Park (#9) and Shakira (#10). Other artists who made the list include Taylor Swift, Usher, Lil Wayne, Kanye West and Paramore.

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

For those wondering how the magazine came up with the list, here's the science: Social music-tracking service Next Big Sound collects data from the MySpace, YouTube, Facebook, Twitter and iLike websites and tallies artists' popularity using a formula blending their weekly additions of friends, fans and followers. The online behavior of these websites' users and the number of artist page views and song plays they rack up are among the data used to determine the rankings.

"The Social 50 provides a weekly snapshot of the artists with whom 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," Billboard director of charts Silvio Pietroluongo explained. "The chart is a logical complement to Billboard's other popularity rankings."

"The volume of online activity that top artists now receive is too big to ignore," Next Big Sound CEO Alex White said. "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 be published each week in Billboard, and also will be available on Billboard.com and Billboard.biz.