NVIDIA's GeForce cards are a dominant force on the latest Steam hardware survey, with 56.7% of gamers using a GeForce GPU of some kind. AMD has lost a little of its steam in the last 12 months, with 25% of gamers using Radeon.
You can see that in January 2015 that AMD had 28.9% of users, but has declined over the last 18 months down to 25%. I'm sure the numbers will increase for AMD, now that it's next-gen Radeon RX 480 is here. In that same amount of time, NVIDIA went from 51.6% to its new lead of 56.7% - all of those users buying GeForce 900 series cards, like the GTX 950 through to the GTX 980 Ti.