Over at MusicAlly they have put together a great list of the top 20 bands on Facebook according to their popularity. 20 most popular music artist fan pages from the chart though, and they are (with number of fans):

1. Rihanna (1.52 million)
2. Coldplay (1.49 million)
3. Lil Wayne (1.34 million)
4. Linkin Park (1.33 million)
5. Chris Brown (1.3 million)
6. Metallica (1.22 million)
7. Pink Floyd (1.16 million)
8. Bob Marley (1.13 million)
9. Red Hot Chili Peppers (1.12 million)
10. Justin Timberlake (1.05 million)
11. AC/DC (997,000)
12. Daft Punk (912,000)
13. Beyonce (906,000)
14. Queen (900,000)
15. Evanescence (873,000)
16. Jonas Brothers (843,000)
17. Radiohead (793,000)
18. Kanye West (792,000)
19. Katy Perry (791,000)
20. Akon (753,000)