To enable Facebook comments widget to be displayed with all blog posts, from the left menu, go to Online site > Preferences. Alternatively, you can type "comments" in the search bar on the top of the page and click on Preferences.
You will be directed to the Preferences page, select the Comments tab from the top panel.
Go to the Facebook Comments section and enter your data.
After finishing all your changes, click the Save button on the top right of the page.