Back in my days as university student, one of the most important concepts in the classes “Project management” and “Investment projects evaluation” was the ROI. With WhatsApp saying To handle 50 billion messages a day the focus is on making a reliable system that works. Monetization is something to look at later, it’s far far down the road.  and Zuckerberg saying Simply put: we don’t build services to make money; we make money to build better services. in the IPO letter, it is clear that not everything is about money.
“Steve’s talked about the goal of Apple, and the goal of Apple is not to make money but to make really nice products, really great products”, said Jonathan Ive .
Yes, not everything is about money, money doesn’t have real value, money shouldn’t be necessary…money shouldn’t exist in the first place
Other unrelated but interesting parts on the WhatsApp architecture article:
– In 2011 WhatsApp achieved 1 million established tcp sessions on a single machine with memory and cpu to spare. (Wow!)
– Ejabberd in FreeBSD. (I played with that!)
-32 engineers, one developer supports 14 million active users. (Good people is worth)
 Inside Apple, Chapter 3. Adam Lashinsky