If you sign in to Vocabulary.com via Facebook Connect, you will get access to an additional leaderboard where you can see how you stack up compared to your friends on Facebook, who are also playing Vocabulary.com.
To see your Facebook Friends Leaderboard, click the "Friends" tab at the top of the round summary panel to right of the question in the challenge.
Please note that when you use Facebook Connect, your email address in Facebook must match the email address you are using with Vocabulary.com.
Read more: Changing your email address
When you are connected with Facebook, you should see an entry that reads "You are connected with Facebook" under the "Connected Logins" section of the My Account page.