LivingSocial, the popular application for sharing lists of your favorite sports teams, beers, or almost anything else, has just surpassed Causes to become the #1 application on the Facebook Platform by total reach, with just over 20 million monthly active users.

According to AppData, LivingSocial was reaching just 2 million monthly active users as of March 25. It’s added about 18 million active users over the course of the last three weeks. LivingSocial has experienced such rapid growth by making good use of Facebook’s “feed form” functionality, which syndicates all application feed stories published by users to their friends’ home page streams, with its “Top 5″ feature.

LivingSocial’s Top 5 has inspired dozens of other applications to follow suit, including top music and film sharing applications iLike and Flixster. International application developers have also adapted the “Top 5″ – for example, friends5 from French developer Sociabliz applies the same user interface to pick users’ top friends.

Will LivingSocial be able to sustain the growth it has achieved and stay at the top of the application leaderboard? Keep following the application charts at AppData to find out.