For more details see the Facebook Reviews extension page.
Attract user-generated content to your Facebook pages. Gain more credibility and engage more customers.
To get the access to the 'AJAX Reviews' settings please go to: System → Configuration → Facebook Reviews
Facebook AppID - a Facebook App ID is a unique number that identifies your app when you request ads from Audience Network. Create a Facebook App first: https://developers.facebook.com/. Then you get an AppID, which you need to insert into this field.
Review Title - specify the title, for example, Facebook Reviews:
Reviews per Page - specify how many reviews will be displayed on the product page.
Chronological Order - set 'Yes' to show the latest reviews first.
Block width (px) - specify the width for the reviews block.
No default styles - select 'No' to set the default style of the reviews block.
Custom CSS - insert here a path to the file to set your own custom style.