Customize → Single
This is single post eg :Single Demo in here you can setting layout of this page
So when you in the Customize then you should click to post detail
Click the Blog single.
You can Enable/Disable Single Top
and can Drag the widgets to the sidebar Single Top
Similar with the Sidebar Single Bottom
You can get a sidebar for only Single page
inside single post , you can setting comment with facebook.
if you want to get notification when some one comment to your fanpage to enter APP IP
You can get APP IP here :https://developers.facebook.com/apps/