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Many web sites are not profitable.

Youtube lost tons of money.

They want to be the market leader and then charge (say for ads), but typically a new service comes out before they can make money.
 
Read up on what happened in the 90's when the web bubble collapsed and you'll understand how many sites like facebook can be valued in the the millions of dollars but be worth almost nothing at the same time.

These companies are only as valuable as the highest bidder is willing to pay.

Many web sites are not profitable.

Youtube lost tons of money.

They want to be the market leader and then charge (say for ads), but typically a new service comes out before they can make money.

This can sum up the entire industry.
 
Facebook doesn't have to make a profit. That'll be a problem for Google or Microsoft to figure out when they buy it for billions of dollars. ;)
 
Like the above poster said, look at the 90's and the internet bubble. Most websites that were destined for fame never made money and are now out of business. Amazon just started making money recently. All those early years were just accounting smoke.
 
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