“In 2011 a fund-raising site called GoFundMe was talking with WePay about the possibility of switching to its service from payment giant PayPal.
Using A/B, WePay could present GoFundMe CEO with an irresistible proposition: Give us 10 percent of your traffic and test the results against PayPal in real time. It was an almost entirely risk-free way for the startup to prove itself, and it paid off. After GoFundMe CEO saw the data on the first morning, he switched half his traffic by the afternoon—and all of it by the next day.”
Wired has a big article on A/B testing.