We haven't tested with K2's captcha. We do work with Joomla's built-in captcha though.
Can you let me know how you have the user registration setup in JFBConnect: Normal or Automatic? If it's automatic, then K2 shouldn't be involved in the registration process. If they are preventing it, then that would likely be a bug you'd have to contact them about. If it's Normal, we can look into it further, but I'm still not sure it's an issue we'd be able to fix on our end since K2 would be interfering with the registration.
Is there a reason you don't want to use the captcha built in to Joomla?
Thanks,
Alex