http://www.worldipv6launch.org/measurements/ has some numbers for % of traffic to major sites (google, facebook, yahoo, akamai, etc) from each ISP that goes over IPv6.

Some major US ISPs: Comcast: 20.61% AT&T: 13.53% Verizon Wireless (not FiOS): 40.03% Time Warner Cable: 3.88% T-Mobile USA: 9.58% Google Fiber: 71.87%

Very large ISPs have been deploying it because it actually simplifies their internal networks (which can be too large for RFC1918 space) and because carrier grade NAT is expensive.