How to remember only a username in Safari?
Go to Safari / Preferences - Autofill. Make sure: User names and passwords is selected.

If you don't want your FB pasword saved, open Keychain Access.

Applications/Utilities.

Select Passwords on the left.

Delete the Facebook keychain. (That's where the password is stored).

Now navigate to the Facebook page, enter your log in data but when prompted for Safari to save that data to a new keychain, click Never.