I recently noticed an email in my gmail spam folder that was from a client--or at least it appeared to be.. it was, but it was actually from "What's APP".. the very popular messenger app used in Europe/Asia and is picking up steam in North America. It went directly to the gmail spam folder because it was considered a 'mass mailer' which it was... you see, when this person installed the app, they had to click on the agreement in order to proceed with the installation. What they were not aware of is that this app would then have access to their contacts (after all, it is a messenger app so it kind of needs access to this) but not only does it have access to the contacts, it would then cross reference all of the addresses with known addresses already in their system and any that weren't in their system, it would send out the mass mail stating that "this person just signed up to 'What'sApp" and so should you!"
LinkedIn did this a number of years ago, but worded it differently to the point that after you installed the app it would say something like "would you like to see if any of your friends are already using this app?" and so you would click YES and it would then send out the mass mailer.. it wasn't really going to show you what friends were already using it, but now that it had sent the mail to everyone in your contact list, you would know who was using it by the fact they would get notification on their app. For those that didn't have the app installed, they would get the email stating "this person you know just installed this app and so should you"... it is very much a security problem and that is why gmail reports it as spam, because this sort of email is very easy to poach and turn into a phishing scam.
So what you need to be aware of is what you are clicking on and agreeing to BEFORE you simply install the app. Just because you have an apple product, for example, doesn't mean you aren't still susceptible to phishing scams and other malware. This does not mean you should not use the app, just be more aware of what the app is about to do when you install it. In some cases you may be able to prevent this mass mailer from taking place by going in to the app's preferences and changing some settings.. Taking a look at one of the FAQ's within What's app and how their response has you changing your settings to full permission is pretty crazy. They do however, go on to explain here, why they need/want access. Most importantly however is paying attention to what the app has access to and what you can do about it BEFORE installing it.
Oh, and if you think What'sApp is bad, you should check out what the Facebook App has access to!