How to Clear Username History From Facebook
Some browsers keep your Facebook username history to make it more convenient for several users to log in quickly. When you click on the username box in Facebook, a drop-down box often appears, listing recent usernames. Users can click on their usernames to save time by not having to retype them. To clear this history, you must delete your browser's history; this information isn't stored in Facebook, but rather as part of your Web browser.
Open Internet Explorer.
Click on "Tools."
Select "Internet Options."
Select "Delete Files."
Open Mozilla Firefox.
Go to the "Tools" menu.
Select "Options," then choose "Privacy."
Click on "History," then select "Clear History."
Open the Netscape browser.
Click on the "Edit" menu. Select "Preferences."
Choose "History." Click "Clear History."