I went to your website and was able to create a user and log in properly. I then logged out and was able to log back in successfully as well. You can see my user with the username of melissa.sourcecoast on your site. Please give more detailed instructions on how to reproduce the error, the actual error text, etc, as I had no problems logging in.
I just tried with Firefox. My account was created and the "Facebook profile successfully connected." properly showed when I was done.
Can you please tell us more about the error you're getting, if you've tested with multiple accounts, if authentication ever worked for you or any other details that may help us narrow things down?
We'll gladly help however we can once we understand the issue a bit more.