OK, this is not funny. I enter my login details on MW3, for Facebook (found in the options menu). It then prompts me with "This account has already been registered. Please try another account."
So I logged on Facebook on my laptop, deleted Call of Duty from my Apps list, and tried again; got the same error.
All the times I try to Login with Facebook, I always get a text from Facebook, telling me that "Call of Duty" accessed my account, and an e-mail reading the same, but MW3 say's it's registered with another account.. Really?