In the past 30 days Facebook referred people to mobile app stores 146 million times

"Facebook has increasingly become a way for iOS and Android developers to grow their apps. In the past 30 days, we have sent people to the Apple App Store and Google Play 146 million times, via clicks from channels such as news feed, timeline, bookmarks and App Center.
Mobile ads are an additional way to drive people to apps. When a person clicks on one of these ads, if they do not have the app installed they will be sent to the App Store or Google Play to get it."

Source:  Facebook Developer blog, introducing mobile ads for apps, 7th August 2012