This is not new: Facebook allows its users to download a file containing all the data it has about them. Today, Facebook ad allow users to upload more Instagram-related data, as well as Facebook to keep tabs on what Mark Zuckerberg's company knows about a person.
Facebook tells you a little more about what it knows about you
Has the American company decided to be a little more transparent? This is in any case what suggests the new measure announced. Facebook will indeed make it possible to consult and download more information than what it already indicated to its users.