jecasc - 11:00 am on Oct 6, 2012 (gmt 0)
Google is only a search engine and there are others.
Facebook provides a service that people want.
On Google I can convince a hungry guy to buy a pizza when he is actively searching for something to eat. On Facebook I have to convince the guy he is hungry first before I can sell the pizza.
People on Google are actively searching for a place to go - Google is not their destination. - exactly because it is only a search engine. If I can convince them with my ad that I am the place to go I can make money and pay Google for it.
Facebook is already the destination. Facebook is the place where people wanted to go and want to stay for the time being. An ad is much less effective here.
That's why Google users are - and will stay for a long time much more valuable for advertisers - even if Facebook has 3 Billion active users.