Can Raspberry Pi be used as a cloud server?

Of course! For $35 a pop, you get 100% dedicated CPU that can be treated like blades in a blade server. The only difference is, it can run out of your house for actually less money than an Amazon EC2 or Rackspace cloud server instance. You only pay once for the hardware cost once, then use your home broadband for serving—which you pay for anyway! Services like DynDNS can give you a fixed domain name even from a dynamic IP. And you get the hand’s-on hardware experience of spinning your own loadbalancers, etc.