Take 2 has announced in its earnings report that 5M units of Borderlands 2 have been shipped. The game has performed well in retail, but Gearbox's use of "shipped" seems to indicate that the title has sold considerably less.
If VGChartz's figures are correct, this is indeed the case. The site Borderlands 2 sales have been on the Xbox 360, with 810K sold on the PlayStation 3 and 320K on PC.that 2.73M units have actually been sold, with 1.57M units sold in the US alone. The site states that 1.6M of the
Borderlands 2 was released on the three platforms on the 18th September 2012.