You all bringing up Femme Fatale like it matters in this discussion. Like Britney wasn't at a different point in her career.
You all set up ridiculously high expectations for PRISM & didn't learn from 3 years ago. Katy debuted at the height of the rise of the digital era & like other artists that debuted during this time, her album buying fanbase in the initial weeks isn't as strong as artists that debuted before this time. Katy, Rihanna, Miley, etc (in terms of looking at pop girls that rose during this time, Gaga being an exception because she's a phenomenon that built up an incredibly devoted fanbase that purchases in masses). These numbers are excellent, and you all can be disappointed while Capitol & Katy throw a party for the huge rise from TD.
Artists like Katy are more focused on longevity & overall impact throughout an era, 1st week is just the beginning.