Facebook should get there, too.
IBM kicked off the mainframe era of computing and to this day is the leader in big enterprise computers and services. Microsoft was an early leader in personal computer software and now dominates microprocessor based desktops and servers. And after joining the scrum at the tail end of the dotcom boom of the 1990s, Google emerged as the leader of the Internet era of computing, amassing huge market share and most of Internet advertising's profits.
Coincidentally, those three companies--each of which dominated an era of computing--are now each worth roughly $200 billion.
Given the history of IBM, Microsoft, Google, and Apple, that each respectively led the mainframe/minicomputer, personal computer, Internet, and mobile eras of computing, it is not that much of a stretch that Facebook, as the company that defines the social era of computing, will be right up there with them. Each of these eras have produced prodigious revenue and earnings, and as Facebook's S1 filing shows, social is already well on its way to stellar revenue and earnings--making the bulk of its money the way Google does, through advertising.
The leaders of each era have managed to lock in a generation of users ranging from business datacenters, PC operating systems and applications, and the portal by which people search. Facebook's social graph will be just as persistent. While other niche social networks will emerge, most of us aren't going to switch.
Of course, Facebook's current revenue and earnings does not justify such a valuation, but Facebook is still young and doubtless will figure out plenty of ways to make more money, including selling valuable new ad units such as sponsored stories against its increasing number of mobile users. So long as it it continues on its existing trajectory, the leader of the social era of computing will join others in the $200 billion club.
Or--dare I suggest?--the $400 billion club.