For the record, they will only include albums released between January 1st, 2011 and December 31st, 2011. So artists like Rihanna only had a little over one month of plays. It's stupid, I know. A 2010 song could have been played 5 million times during 2011 and they would have disqualified it.
They don't count any song that wasn't released in 2011, so that's why Katy isn't on the list and Rihanna is so low. They only count new songs (meaning it couldn't have been released on a non-2011 album) and albums that were released in 2011. So even if it was released on December 31st, 2010 and received 5 million plays, they disqualified it.
I don't get Last.fm; it says Rih's not even the top 10 in these lists but I looked her up and her total listeners (whatever that means) is higher than pretty much everyone, even Adele, Gaga and Britney. How does this work.