I actually was thinking about FF's performance the other day, and I realized that it wasn't as bad as people make it out to be. I see all these comparisons to Beyonce, Rihanna, P!nk, Kelly albums but Britney is at a FAR different place than they are. Beyonce has the support of her R&B audience [which has always been a loyal one! Just look at Mary J. Blige]. P!nk and Kelly both have support from Adult Contemporary, which are constant album buyers as well, and Rihanna has tremendous crossover appeal which boosts her album sales, too.
Britney, on the other hand, has always been at the mercy of Pop radio and the Pop audience ONLY. The Pop audience is notorious for being extremely fickle in who they support and how they approach buying an album.
Just my 2 cents. Gaga as well is another artist who really doesn't have crossover appeal and is also at the mercy of Pop radio, but promotes enough to scan enough albums to come off with a solid # in the end. Whereas Britney who hasn't initiated a massive promotional campaign in 10 years.
In other words, FF was lucky to sell what it did.