"Prove" is a pretty strong word. I looked for and didn't find a lawyeresque privacy agreement, but even if I did that's not proof either since they could lie. They could invite you to their data center, show you their code, etc, but that is still not proof since they could be hiding things from you, or simply wait until you leave to flip the "log all data" switch.

It always eventually comes down to trusting a company, or trusting strangers. You're already doing this, because you are viewing this on some form of a computer that you didn't completely hand-build, and are running software that you didn't build from scratch using your own, self-built compilers.

I say a potentially better way of looking at this problem is: I know Comcast logs my DNS queries and who knows what else. I however don't know that this site does. From a pure privacy standpoint, I have nothing to lose by switching over and everything to gain.