On A US Scale:
Britney Spears - Her album sales were lackluster, sadly.
Beyonce - Her sales were decent, but did not compare to her other eras.
Lady Gaga - She had good sales, no denying. But if you take away the discount (no shade) and the fact that it was one of the most hyped albums in a while as well as the "Album of our Generation/Decade" claims (don't remember which), they were just average.
Rihanna - While she's had minimal promotion because she's too busy smoking Blue Ivy Marijuana in Hawaii and still manages to be in the top 5 on Billboard, she's only at around 600k in US sales at the moment. We know she's a long-term sales chick, but she could be doing better.
The four biggest pop stars that had albums in 2011 all in a certain way lacked the huge potential they were being held up to. They all had either big singles or big promotion in one way or another to get their album's names out to a point. Then, you look at Adele, who has only the VMAs listed as a "huge" U.S. promotion spot and a few talk shows, and completely has destroyed sales, records, and apparently eras. Was this because of an internal issue of the pop stars, or was this because of who was by-far the biggest artist of 2011 and who may also become the biggest artist of 2012?