But wouldn't the same user a fool as well using FB connect as a login to all the forums and marketplaces? If one manage to crack his FB account, then go to the apps list, and sure the hacker has access to all of the forums, marketplaces, and other sites that uses FB Connect.

On the other hand, if you use one email and same password across all sites (but different email password), even if the hacker hacks the email, the hacker does not have the access to forums, marketplaces, etc. The hacker would have to hack each individual sites.

I suggest a thorough course in security threat analysis and attack surfaces.