login issue

There are a couple of settings in the General section of the JFBConnect configuration area that should help you to achieve what you want:

How can I associate a username automatically (by Email), and if the email does not exist in the system the system would ask the user to register.

1. Automatically Link Users by Email - If a user logs into a social network for the first time, and the email address is already in the system for a Joomla user, then it will be mapped to that account.

Is it possbile to skip registration as well, meaning the user will be registered automatically based on the Google profile?

2. Enable Automatic Registration - JFBConnect will attempt to automatically create a user account. Some social networks, such as Twitter, may not provide an email and the user will be directed to the screen you are seeing to either login/link an existing account or create a new one. Google is one of the networks that allows email address to be imported, so the setting should work for you.

Try these settings and this should get you closer to the registration flow that you want. Of course, if you have any issues, please let us know.
-Melissa