Maybe not addicted but she has a habit of going to Facebook every time she uses her phone.
You could make the argument that the content is worse in some way but even that is debatable since depending on who you follow on FB (groups, etc.) really changes what the content is.
"Despite adverse consequences" is a key component of addiction. For instance, many people can consume alcohol fairly regularly without the alcohol causing trouble for them. But if the alcohol starts to get in the way of work or disrupt your family life, that is a textbook warning sign of alcoholism.
If her compulsive use of facebook is getting in the way of things she wanted to do instead, that sounds an awful lot like addiction.
The way I quit was to log out. I compulsively kept opening a new tab but having to log in was enough to make me close the tab. The compulsion vanished much quicker than I thought. In a week or so Facebook was out of my life.
If a habitual behavior is bad enough to interrupt somebody's ability to accomplish routine tasks, then it's pretty conclusively an addiction.