Mark Zuckerberg

Every time you log in to Facebook, every time you click on your News Feed, every time you Like a photo, every time you send anything via Messenger, you add another data point to the galaxy they already have regarding you and your behavior.

The Gmail app is definitely the app I use the most. I am always running from meeting to meeting, so it keeps me up-to-date with everything going on. I actually e-mail more often from my iPhone than my laptop, so having a nicely designed e-mail app is really important.