Yes, that's what I meant when I said "mb", perhaps 95th percentile billing would have been more explicit. And when you start dealing with Facebook or Youtube sized loads, you generally start peering rather than just buying transit from various providers.
When you peer, you no longer pay a 95th. You work out a peering deal with some provider you send a lot of traffic to and effectively connect directly to them in some shared facility. Some providers will charge you a 95th rate when you peer, but even then it's vastly cheaper than what you'd be paying for normal transit.
Generally with CDNs you get charged per GB until a certain amount. Then it'll depend on your specific contract whether its GB or Mbps; there's no hard and fast rule.
Building your own inftrastructure doesn't make bandwidth free, it just eliminates the cut that the provider normally takes.
Also, Akamai doesn't charge by the megabyte either, they charge by the megabit per second, as does most anyone once you start dealing with Facebook or Youtube-size loads.