Google is valuable because they are the point where people declare their intent on the internet, ie "I want to buy something so I will search for it." And then get advertisements based exactly on what you are looking to buy. Or what you are looking at in your gmail account, etc.
Facebook is considered to be so valuable because all the profile information, status, updates, etc builds a very complete picture of you to potential advertisers. And they have mind blowing traffic statistics. That being said, I do think the value of facebook is largely speculative. But the potential is certainly there.
PD
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