Cloud Foundry: Open source PaaS serving millions of users

The millions of users claim comes from the fact Cloud Foundry powers the Comic Relief (charity telethon) payment system, processing 400 credit card payments a second during the peak evening.

Add to this that it's used by General Electric, Ford, Huawei, the US, UK, Dutch and Australian governments, and many more, plus the public PaaS offerings of Pivotal Web Services, IBM Bluemix, Anynines, Swisscom and more... There's a lot of usage.