This is tough. First of all, it doesn’t help to judge her for it. To make her feel guilty or to berate her for phone use is not helpful. Second, just because she’s doing a lot of screen time, doesn’t mean she’s necessarily an “addict”. Addict is a strong word. The question is really how she’s using that time, what she’s using facebook for, etc.
There’s no easy solution, but if it really feels like there’s a problem here, it can help to refuse to speak with her while she’s looking at her phone. If she tries to do this to you, just stop talking, or say something like “I’ll wait 'till you’re done.” Also, it can help to inform her of your own feelings. Tell her how it makes you feel to see her do that. Tell her stories about your own realizations, “I used to be on facebook all the time, but at some point i realized ____ and now I feel ____”
You can also try prompting her on how it makes her feel. Asking her open-ended questions. Is it really a problem? Maybe there’s no problem. Or maybe it’s related to another problem, i.e. she’s escaping into her phone because she’s taking the wrong classes and bored out of her skull, or because she’s feeling sad/anxious/depressed.
Other useful prompts: does she ever regret using it? At the end of a session, the end of a day, or the end of a week, does she feel guilty about how much time she’s spending there? If so, getting her to notice that herself can help.
Just some thoughts - hope they’re helpful