Yahoo claims 500 million users around the world at a time when Internet advertising revenues are growing industry-wide. But Yahoo isn't doing anywhere near as well as Google. Recent management shakeups are part of Yahoo's struggle to capitalize on its big user base.
There will be challenges for Google and Yahoo. Both face more competition from MSN and AOL.
And there is a growing chorus of critics who say that all the personal information being gathered by these companies is infringing on individual privacy.
It's also possible, in the still young and dynamic Internet marketplace, that a company could suddenly emerge with a new and better technology that nobody can foresee.