It’s so much more fundamental than that.
So many UX designers clutter apps with badges, virtual currencies, etc. because they don’t understand that the app itself is the game.
A game is a form of structured play. Play is a voluntary experience that people pursue for the sake of having that experience. Structure is the set of rules, limitations, and win/lose conditions that push people to a goal.
Facebook is a game. Twitter is a game. Instagram is a game. Even ebay is a game.
Facebook, Twitter, and Instagram’s core mechanic is trading. You trade your attention for someone else’s content. The win condition is getting the most attention. You don’t have a leveling system, a plethora of different badges, or any of the other usual contrivances that designers employ in getting-things-done and fitness apps. You have interaction with *real* people. That interaction is constrained to viewing, liking, commenting, and sharing - and it works so much better than artificial challenges.