If you want iPhoto to forget you even have a Facebook account—maybe because you want to sign in using a different Facebook account—choose iPhoto > Preferences and then click the Accounts button. Find your Facebook account in the list of accounts, click it, and then click the minus (−) button beneath the list. You will be asked to confirm the account deletion.
When you remove a Facebook account from iPhoto, it doesn’t change what’s posted on Facebook, but iPhoto does put any photos from your Facebook albums that you haven’t previously imported into iPhoto’s Trash. Luckily, you needn’t root through the trash for your discarded treasure: iPhoto offers you the opportunity to import the photos before deleting the account.