To continue on the topic of WhatsApp, here’s a post on why ads never appear in the service.
No one wakes up excited to see more advertising, no one goes to sleep thinking about the ads they’ll see tomorrow. We know people go to sleep excited about who they chatted with that day (and disappointed about who they didn’t). We want WhatsApp to be the product that keeps you awake… and that you reach for in the morning. No one jumps up from a nap and runs to see an advertisement.
I fully agree on their mindset and them channeling their resources and focus on something that matters to the customers, not the advertisers.