In the past, the Silverado 1500 has outsold the F150, as a large portion of F-Series sales were the Super Duty. It's only the last couple years that the F150 has started outselling the Silverado 1500.
I didn't realize the Tundra was that strong. I assumed it had pockets of the country that it sold well in while others that you couldn't give one away. Around here Ford is by far and away #1 in sales but I see as many Tundras as I do Chevy/ GMC trucks. But this area loves the Tacoma so I would assume that if someone with a Tacoma decides to go larger they will go to the Toyota dealership first.
Add the Sierra to the Silverado and see what happens...GM has a split brand, Ford doesn't.
In the half-ton class, yes, GM still outsells Ford. But when you include 3/4 and 1 ton classes, Ford outsells the combination of Chevy and GMC.
March 2013 Top 15 Pickup Truck Sales - PickupTrucks.com News
And, IIRC, in the 3/4 and 1 ton classes, Ford outsells Chevy, GM, and Ram combined.