Do you need more than $10M/year for 450M users? As I've said in another comment they barely stored anything server-side and it's known they have a very optimised stack
What stops me from getting together a few friends and stealing your business model?
At some point you have to add value just to keep the wolves at bay.
I'd like you to read this and contemplate.
http://www.catb.org/esr/writings/unix-koans/ten-thousand.htm...
If you didn't bother reading and contemplating, here's the gist:
There is no wolf to fear. We are all finite beings with finite time and resources. You 'stealing' his hypothetical business model doesn't really accomplish anything except tie up your resources retracing his steps. You would be the fool reimplementing the pipe.
Now if you are the type of person who reduces life down to only pursuing the almighty dollar rather than trying to solve difficult extant problems... Well... Maybe that would be worth it to you. Even then though, he has already captured his audience, and he'll likely have a reputation advantage over your copy.
Anywho. Food for thought.
That's not how it works. I'm pretty sure you and I could write a Snapchat clone in a weekend. This doesn't imply we'll ever reach 100M users.
If we assume they maybe averages 75% of that over 6 hours, that's 146Gb/s * (60606)s / 8bytes/byte * 0.75 * 0.05dolars/GB = $14,782.50 for a 6 hour period (estimate).
That's highest listed bandwidth tier price for Azure, I'm sure they would pay less for a number of reasons. But let's just say that $10k for a whole day may not be out of the realm of possibility. That puts bandwidth costs possibly North of $3 million a year. Even at $1 million, that's a lot of money.
And if that's audio/video Vs just text, clever NAT hole punching techniques could reduce it if truly needed.
But, it seems WhatsApp was on SoftLayer? In that case their costs might have been vastly higher.
[1] http://web.mit.edu/e-club/hadzima/how-much-does-an-employee-... (This is old but nothing really substantial has changed except.)