Google and Facebook have massive data centers to handle the load. Each data center contains thousands of servers. A data center is basically a huge warehouse filled with servers. The load from searches, e-mail, etc., is distributed among thousands of servers. One server could not handle this.
I'm a few credits away from a Computer Science Bachelor's, and this information is from professors whose entire life's work is research on solving these problems.
WoW's servers are connected, how do you think they do cross-realm grouping?
Also, what about time zone differences, names, regions? WoW has datacenters throughout the country to reduce latency. Or should everyone in NY connect to a server in California? For all you know, each server is actually one of several virtual machines on one physical machine. This is likely the case, many companies do this to save costs.
No large company uses one server, you just think they do.