It's the Facebook button itself, and Facebook keeps adding that bug back in. It's already been reported to Facebook by multiple users and we're hoping it's fixed soon.
Instead of using the Javascript form of the button, you can use the new settings in the SCLogin module that let you specify a unique image for the Login with Facebook button instead. That actually is a little faster to load (since you're not using Javascript) and it's not prone to bug-introductions when Facebook updates their JS library.
Hope that helps, but if you still need assistance, just let us know!
Alex
JLinked button causing a containers to enlarge prior to loading
