FourFiveSeconds sold more than half of what PI is predicted to sell in a week in 29-30 hours, the comparison makes no sense if that's the song you were hinting at. Not to mention the song was nothing like Rihanna has ever released before but it still went on to sell more than a few of her #1s.
The performance of PI isn't surprising at all. This was honestly all expected as soon as the "Born This Way" era began. Gaga will never recover unless she records and releases a song that she didn't have anything to do with behind the scenes.
Another drop like that would mean #26 by the next update 
The second week drop will be insane.