Have just read a very interesting piece on edgeio which has noted what appears to be a flattening trend on the traffic to the big portals. Essentially, since 2002 there has been a gradual reduction in the competition between the dominant portals, suggesting, possibly, that web users are turning their backs on the "portal experience".
I have to say I think the argument is compelling. The portals of the world are now established, and it is highly unlikely that any players would start spending large amounts of money to enter such space. The only genuine revenue streams in portals are ads, and most web users are starting to block such delivery.