Everyone loves Duolingo right? I think they should have a leader board of points of people whether you friends with them or not. It gets a lot of people motivated to be at the top. Also, there needs to be more ways to get and use lingots.
An overall leaderboard would just highlight the very thin slice of users who are (1) doing a large number of course quiz-outs in a short period of time (2) cheating by scripting. Even if those were somehow excluded, I don't know how motivating it would be to the average user to see that there are users who each and every day legitimately rack up more XP than they earn in a month. People don't tend to respond all that well to manifest inequality.