Leaderboard should have a weekly or monthly ranking as well
The current leaderboard shows your total score for all time. To promote people continuing to practice, there should also be a weekly or monthly leaderboard. That way, even if you started later than your friends, you can still compete with them to have the best weekly score, and people with high scores will have an incentive to keep coming back.
As an example of a similar system, Stackoverflow displays the weekly leaderboard as the default, which gives new users who have had a good week the chance to show up along side the most dedicated long term users.