Build their own? Probably not, unless they are doing it at massive scale and the cost of replacing parts beats out buying a server with a 3/5/7 year warranty.
If you have static usage, or at least a base load owning your own hardware to support it does make sense though, colo space is cheap, the biggest disadvantage from a financial perspective to owning your own gear is you have a depreciating CapEx instead of a 100% deductible OpEx.
Real investors care about cash flow, which means the difference between purchasing/depreciating and leasing/renting(OpEx) is negligible. Whats far more important is the business benefits of the best approach for your business.
I don't know what the current state of things is, but 5 years ago they were doing a mix of buying off the shelf and building their own supermicro-based systems.
