The difference in this particular case is that Whatsapp made an explicit promise that they would not start sharing their data.
Respect for your privacy is coded into our DNA, and we built WhatsApp around the goal of knowing as little about you as possible: You don't have to give us your name and we don't ask for your email address. We don’t know your birthday. We don’t know your home address. We don’t know where you work. We don’t know your likes, what you search for on the internet or collect your GPS location. None of that data has ever been collected and stored by WhatsApp, and we really have no plans to change that.
If partnering with Facebook meant that we had to change our values, we wouldn’t have done it. Instead, we are forming a partnership that would allow us to continue operating independently and autonomously. Our fundamental values and beliefs will not change. Our principles will not change. Everything that has made WhatsApp the leader in personal messaging will still be in place. Speculation to the contrary isn’t just baseless and unfounded, it’s irresponsible. It has the effect of scaring people into thinking we’re suddenly collecting all kinds of new data. That’s just not true, and it’s important to us that you know that.
Make no mistake: our future partnership with Facebook will not compromise the vision that brought us to this point. Our focus remains on delivering the promise of WhatsApp far and wide, so that people around the world have the freedom to speak their mind without fear.