I just wanted to verify that I could reproduce the behavior that you're reporting. However, I just created a user on your site with my Facebook credentials and received the new user email.
Hello Melissa Lynne SourceCoast,
Thank you for registering at BuyPostSell.com.
You may now log in to buypostsell.com/ using your facebook credentials.
I'm just not seeing the behavior that you're describing. Please make sure you're attempting to log in 1) with a user that does not already have a FB user mapped to an existing account and 2) does not share an email address with an existing user on your site. If this is true, then you will need to provide detailed step-by-step instructions on how to reproduce on your site.
-Melissa