This plugin simply adds required open graph tags for facebook share to your site. Facebook uses those tags to get a short description and a thumbnail of your site or post. Below you can find useful information about plugin usage.
Default Logo URL
Simply put a logo link to this field eg:
http://shailan.com/shailan_logo.png. When a user likes or shares your home page this image is used. Also it is used if there are no featured images on your posts. So be sure to add a featured image to your posts if you don’t want to display this image on your post shares.
This option is required for certain pages like your home page. This field is optional however. If you don’t know your facebook id you can find it here. It is located in a box named Admin under “Step 2 – Get Open Graph Tags”.
Using this field you can put your Application ID to your site header. If you are not using your site as a Facebook App this action is not required. You can get Application ID for your site here.