We've never tested with CBSubs before. Until earlier this year, that was an encrypted extension, which made it impossible to diagnose any issues with. Since it became open-source, we simply haven't had any questions about it. I know that users have made JFBConnect and CBSubs work together before though, but it may be part of the problem you're having.
With that said, the main reasons that a user isn't created in CB when registering through JFBConnect is if you have required fields set that aren't being imported into. If there are required fields that don't have any value, CB will reject the user and not save them. I would check your CB Fields and make sure that *no* fields are marked as required that don't have a 'mapping' setup in our Profile area. JFBConnect won't show all the fields from CB, since we can only import into certain field types (text, textrarea, birthday, etc). If you have other fields (radio, select, etc) marked as required *or* a text field marked as required but not 'mapped' to a Facebook field, that is likely the problem.
The easiest test is to go into CB and make all fields "not required". Then, test again. If that works, start enabling the "Required" setting on a few fields at a time until it breaks again and then investigate the problem.
Alternatively, you can switch to "Normal" registration and register that way, since you'll be forced to fill out the fields during registration. That's also a viable option if you have to have some fields marked as required that we can't import into.
If none of that helps, I'd say to try disabling CBSubs (temporarily) to see if that fixes the problem. If so, hopefully we can help you re-enable it.
Hope that all helps get you started, but if you still have issues, let us know.
Thanks,
Alex