Getting username from Facebook id

If you have a facebook id or username, you can get their username or facebook id respectively by appending it to the end of this link:
https://graph.facebook.com/

For example, if you wanted to find the facebook user with the id of 10 [the lowest it can get], then just go to:
https://graph.facebook.com/10
Which gives you: marcel

If you wanted to get the facebook id of a facebook username, then just add their username to the end:
https://graph.facebook.com/marcel

Which gives you: 10

[both links output the same result...]