What is a Facebook fb:app_id and why do I need one?
Facebook provides analytics tools that show all referral traffic to your domain from Facebook including when people link to your site in their Facebook status messages or click on social plugins such as the Like or Share button, etc. This is called Facebook Insights, and it can help you optimize your site and its content for sharing on Facebook.
To begin using Facebook Insights for your Largo site you need a Facebook App ID. You'll then copy and paste the App ID into your Largo dashboard in Appearance > Theme Options > Advanced. In the Site Verification section, look for the Facebook app ID meta tag field:
How to Get Your Facebook App ID
Facebook has a very useful article on how to get your Facebook App ID. If you don't yet have a Facebook Developer account, you'll first need to create that here on Facebook. There are only a few simple steps to create a developer account, then you'll be able to copy your App ID.
After adding your Facebook App ID to Largo, you'll begin to have access to data from Facebook Insights. Here's another Facebook article on how to access Insights.