Microdata is a WHATWG HTML specification used to nest metadata within existing content on web pages. Search engines, web crawlers, and browsers can extract and process Microdata from a web page and use it to provide a richer browsing experience for users.
The simplest explanation for this is that Facebook uses that data to make money. No, Facebook doesn’t sell your data. But it does sell access to you, or more specifically, access to your News Feed, and uses that data to show you specific ads it thinks you’re likely to enjoy or click on.