The way you framed the question, it seems so obvious that they "had to find a way to monetize". But there were probably some inflection points. This is where the issue of the customer trusting the website/app comes in. Initially, WhatsApp was charging a very small fee ($1 a year?) and making it appear as if they were doing it to avoid selling customer data.

The following post has already been linked many times during the related comment threads, but I am posting it again because I bet most people never actually read the whole thing.

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Link:
https://blog.whatsapp.com/245/Why-we-dont-sell-ads

Text:

When we sat down to start our own thing together three years ago we wanted to make something that wasn't just another ad clearinghouse. We wanted to spend our time building a service people wanted to use because it worked and saved them money and made their lives better in a small way. We knew that we could charge people directly if we could do all those things. We knew we could do what most people aim to do every day: avoid ads.

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.

Advertising isn't just the disruption of aesthetics, the insults to your intelligence and the interruption of your train of thought. At every company that sells ads, a significant portion of their engineering team spends their day tuning data mining, writing better code to collect all your personal data, upgrading the servers that hold all the data and making sure it's all being logged and collated and sliced and packaged and shipped out... And at the end of the day the result of it all is a slightly different advertising banner in your browser or on your mobile screen.

Remember, when advertising is involved you the user are the product.

At WhatsApp, our engineers spend all their time fixing bugs, adding new features and ironing out all the little intricacies in our task of bringing rich, affordable, reliable messaging to every phone in the world. That's our product and that's our passion. Your data isn't even in the picture. We are simply not interested in any of it.

When people ask us why we charge for WhatsApp, we say "Have you considered the alternative?"

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Notice the last line. And ask yourself if you were one of those people who actually believed these words when the blog post was published.

This is why we really need to be outraged. It is surprisingly easy to say the kind of words that con people and then take advantage of them over the long run. And then we make it worse by acting like it is no big deal. Acting like it is no big deal actually emboldens such companies. Now clearly FB lacks this moral compass right at the top (and yes, all my comments make this pretty obvious), but I am starting to wonder if the companies are getting away because there are absolutely no negative sanctions. Soon, this will just become the norm and the accepted practice. If LittleStartupCo pulls a similar bait and switch tomorrow, they will say "Yeah, but WhatsApp did the same thing, and there was a small commotion and people just quickly moved on"

Don't move on. Actually create a ruckus and cause some backlash. If the only problem that the company faces is a few nerds making a small ruckus on a nerd forum, then they will keep doing these things.

Instead, next time you see your friend who works at Facebook/Google/Microsoft/... talk about ethics in any context, mock them for participating in the discussion when they clearly don't have the spine to display the same ethics in their professional work. Just automatically discount their views/doubt their motives in every context, and I bet you will see the message will slowly start moving up to the top of the org.

Even better, stop hiring alumni of these organizations unless they make an open statement that they were sorry for their involvement in organizations which had no regard for privacy. Does it sound drastic? Then how does it feel to have supported the rise of WhatsApp in their days when they badly needed the support, only to see the bait and switch and the oh so casual - "sorry, but we are not really sorry, just FO losers"?