8. Skype
Skype, which Microsoft bought for $8.5 billion in 2012, has been downloaded over a billion times on smartphones. The app got a face lift in 2017, which has helped it better compete with the likes of Apple's FaceTime and Facebook Messenger.
7. Snapchat
Snapchat launched on the App Store as Picaboo in 2011, and is now one of the most popular apps of all time. As of February, Snapchat had 191 million daily users.
6. Google Maps
Google Maps is perpetually among the top 10 apps in the App Store, and is the sixth most-popular app of all time.
3. YouTube
YouTube is the most popular of Google's iPhone apps. The video platform is also consistently the top free iPhone app in the App Store, and is the third most-popular app of all time.
2. Facebook Messenger
Facebook's Messenger app is a messaging app spun out of Facebook's in-app chat service. The app has become immensely popular, and is consistently near the top of the App Store's free apps chart.
This year, Facebook is giving Messenger a redesign to make it look less complicated and cluttered.
1. Facebook
Despite a tumultuous 2018 so far, Facebook remains one of the top iPhone apps in the world, and is the most downloaded iPhone app ever.
In the first quarter of the year alone, Facebook was downloaded an estimated 29.4 million times.