Hotmail, Gmail et al, how do they do it?

I did my normal once a month check of gmail today and noticed I have 4.6 GB of space and it is growing every minute.  We are moving to 1 GB mailboxes at work and that is going to chew up huge amounts of our most expensive storage as everyone classes email (of all things) at mission critical.  A lot of people are already complaining about 1 GB not being enough and asking for double if not triple the size.  We have less than 10,000 mailboxes so this leads me to gmail and similar organisations.  I don't have a clue how many people use gmail and how many actually use that space.  But lets say there are 10 million people using 4 GB.  Thats 40 PB.  How is that possible for free?  Does advertising revenue mean that much?

Lets say it costs $20k to purchase 5 TB of semi decent storage.  40 PB = $16m.  Thats not revenue, thats outlay.

If my estimation is anywhere near correct, then individual companies that provide hotmail/gmail/yahoo/etc are probably the biggest single user of storage in the world.   I would love to be the storage vendors rep for those accounts.  I somehow doubt that any USP V or DMX4 systems would be used for web mail but a sales dollar is still a dollar.

I would love to know what they use and especially how they cope with loss of data.  But then again, is there any reason to protect it?

I googled gmail storage vendor but found nothing of value.  I wonder is webmail type email the future of corporate email?