This graph is comparing mobile users to desktop users. In this graph you can see that as time passes and technology develops more people stop using Facebook on their desktop but people downloading the Facebook app on their smart phones has increased. As you can see in this chart it is comparing how over the years people got out of using their desktops to using smart phones which is way easier to use to by the way!
On this table we see the that there are more mobile users than desktop users over the years.