To link your Facebook account to your existing IMDb account, please go to the IMDb Linked Accounts page. Click the "Link your Facebook account" button to connect your IMDb and Facebook accounts.
To unlink your Facebook and IMDb Account, please go to the Facebook > Account Settings > Applications page within your Facebook account. Find the IMDb application and click the gray "x" to the right hand of the application to remove the link. Once you've taken that step, you'll still have an IMDb account, and a Facebook account, but your accounts will no longer be linked.