Where do companies like facebook, google, yahoo etc generate revenues from?

[Stabbers]

All these websites are free to users but are billions of dollars worth and its not that they can sum up the amount through advertising at just one corner. Is it only through shares or something else?
But, what about the beginning stages where they wouldn't have made a mark at the stock market...

[SONNEH]

They are worth billion of dollars, that doesn't mean they have to be covered buy it, the revenue from the ads pays for the the servers and workers. The rest is revenue. They're worth so much because they have the capacity they have to generate so much many with all the traffic they bring.
In the beginning stages they needed to have a good idea to open up to people, most importantly wealthy investors. It's like an internship from the start, you need to take a risk to make it work.