One of the unusual characteristics of the computer industry in recent years is the rapid rise of companies to almost complete market dominance of their respective sectors.
Things began with Microsoft, whose Windows operating system is still unchallenged on the desktop. Then came Google, which more or less owns the online search world (with the notable exception of the important Chinese market), and after that Facebook, which is probably hurtling towards 800 million users at the moment. What this means is that it is almost impossible for other companies to enter those particular markets and compete against the incumbent.
On Open Enterprise blog.