Facebook paid 16B Dollars for 400,000,000 active phone numbers and all the personal info bound to them. It's not the app itself it's the data, you know it well too.
However, they wouldn't have had 400 million active users, if they didn't make WhatsApp ubiquitous. And with only half or a quarter of the users, they'd be worth far less.
It's likely most of those users were once on Facebook, so I don't think personal info was a big selling point here. I think they made the purchase because 400,000,000 users outside of their bubble is a little worrying. It's a threat, what happens if WhatsApp begins to pivot and push more into Facebook territory?
That's not the case. WhatsApp barely holds any data. The user doesn't have to give over their personal details. It ties them to their mobile number & messages are stored only until they're pushed to the client. Once pushed they are then removed from WhatsApp servers.
If you map some 5-10% of these phone numbers to real users (which facebook can do), then you can map almost all of the remaining phone numbers to their real names as well, using the graph info.
There were some recent papers on algorithms how to do it, and they managed to match ~90% of 'anonymous phone numbers based on the facebook graph data. You stay anonymous only if all your friends and their friends care about anonymity as well.
even if removed, WhatsApp could store metadata like who is talking to whom. This is valuable to try to identify the trendsetters and key people to market to. With a bit of text analysis like google do with gmail, they can personalise ads more. all very very valuable data.
