[iso947]

Whatsapp contents are secure, but who you are talking to - what groups, what time, how often, is all valuable information.

If you are in a group with a lot of Facebook users that have elementary school kids but are otherwise unrelated, chances are you have one too. Facebook might even infer who you are, and your kids, even if you don’t have an account

[cassianoleal]

Not to mention that most users continuously sync their contact list, and with that info FB can create a much broader and more precise graph of all the network connections. Cross that with, say, political groups you're part of, and you're not only exposing yourself but everybody else who's close to you, regardless of them using either app.