How do I link my Facebook account to my IMDb account?
First, change the email address on your IMDb account, so it is the same email address registered on your Facebook account.
Then go to the IMDb Linked Accounts page. Click on the 'Link your Facebook account' button to connect your IMDb and Facebook accounts.
How do I unlink my Facebook account and my IMDb account?
You can unlink your Facebook account and your IMDb Account by going to the Facebook > Account Settings > Applications page. Find the IMDb application and click the gray "x" to the right hand side of the application to remove the linkage between IMDb and Facebook. Once you've done that, you'll still have an IMDb account, and a Facebook account, but the two will no longer be linked.
If you want to also delete your IMDb account, you can do that here.