The friends ranking is displayed in the "Friends" area on the right column of the Home section. When one of your friends passes you in the leaderboard, you can receive an email (if enabled in your notification settings) notifying you and encouraging you to keep up with them.
There are three different rankings:
- This week: Ranking based on XP obtained since the start of the current calendar week;
- This month: Ranking based on XP obtained since the start of the current calendar month; and
- All time: Ranking based on XP obtained since the user began using Duolingo.
The friends ranking can be found in your profile. Just below a button labeled "INVITE FRIENDS" there is a section called "Friends". Only the All time ranking is available.