The numbers aren't into the millions like her first three albums were, within the first couple years of their release... but I don't think 638,000 copies sold in the US is embarrassing, when you consider Stevie's age, the average age of her core fanbase, and the fact that she's female (not trying to sound sexist, but with only a few exceptions, female artists always sell less than their male peers).
Stevie's doing better than her female peers (Pat Benatar, Heart, etc.), and she's even doing better (or least comparable) numbers to some of her male peers.
For example, Paul McCartney's 'Driving Rain' is "only" gold, and Tom Petty And The Heartbreakers' 'The Last DJ' hasn't even been certified gold yet.
As usual, sales figures are not necessarily indicitive of quality, but in a discussion of sales, I don't see how Stevie's numbers are anything less than impressive, when all factors are considered.
However, I would expect bigger numbers from
Fleetwood Mac... especially considering the additional promotion they received, and the breadth of this tour.