In March 2018, amid story after damaging story about data-gathering and electoral manipulation using social media and increasing concerns over misuse of Facebook data for political purposes, a graphic that appeared online in March 2018 brought attention to how that information is tallied:
So apparently Facebook has you labeled as either Very liberal, liberal, moderate conservative or very conservative.
To view it you can go to settings > account settings > ads > your information > your categories > US Politics:
