Amazon recently announced their Virtual Private Clouds (http://aws.amazon.com/vpc/) for enterprises that will not keep a customer on multi-tenant infrastructure if they wish otherwise.
In any case, except for Google/Facebook-scale data and content providers, having your own datacenter for 25m users sounds premature. Many large enterprises use private clouds hosted and managed by service providers like IBM or Verizon; this makes sense when their core business is their service, not the infrastructure itself.
