Multiple devices and linking them to a single person is an active area of development. Say your phone you’ve never signed into Facebook on safari, so Facebook w

Multiple devices and linking them to a single person is an active area of development. Say your phone you’ve never signed into Facebook on safari, so Facebook won’t know what account it is. But you sign into some other service that you give it your phone. Since phone is pretty uniquely identifying, the two companies share info and thus Facebook knows that safari browser is you and can track everything you do to your account.