I know someone who has, but I'm not sure if I'm allowed to say who. They're a really big site and you'd probably know the name, but they're doing 1000's of requests per second.
But other than them I don't know of anyone else.
My company is built totally on Lambda, but we haven't launched yet.
I see some people using it for small use cases and it worked fine. In particular "I need to run this tiny integration function somewhere". However, I would be really careful to do something that has huge traffic.