Numbers time...
631,000 sales on day 1.
926,000 for the whole first week.
A 77% drop presumably equates to real sales of 213,000 (rounding to the nearest thousand like the the others obviously have been).
But once you take out the pre-orders and stampede of day one, they only sold 295,000 in the rest of that first week. That's not a bad drop-off at all, then. It's still not that simple to find stock on shelves; it was sold out on 360 everywhere I went last weekend.
Not that I can justify buying it, having just got a Wii and WiiFit instead, but I'd like to play it at some point.