*** Update: this issue has been resolved in latest release ***

The new username feature from Facebook was released after Signature was released in the Apple Appstore. Signature has been modified to also accept Facebook usernames. The updated application is currently under review with Apple and will be available soon.

The current workaround to finding your numeric userid if you have a username is as follows. Click on one of your friends (Hint: choose someone who doesn't have a large friend list!). Look as his/her friend list and find yourself on that list. Make sure you click on All Friends and not Mutual Friends otherwise you won't find yourself in the list. Hover over your picture and look at the link in the status bar at the bottom of the browser. The old style link should be shown with your numeric id there.

If you are using a browser that does not display the url when you hover over your picture. Right-click and select copy url and paste it into a text editor.