This is not a case for MongoDB, but ServerDensity and their blog shares some great insights about scaling databases to a massive load. Their experience is similar to StackExchange, and they use bare metal servers quite extensively for the best possible performance.
Throwing more hardware at a database problem isn’t always the solution. Some systems suffer from poor database performance due to bad queries, ORM libraries, network bottleneck, or a number of potential problems. But it’s amazing how things like storage technology has advanced to enable greater performance in smaller packages. Our company is evaluating Nimble Storage solutions right now, and I’m impressed with their secret sauce software.