There have been other dominating monopolies before, (ma bell telecom monopoly comes to mind) which was much dangerous than a website.
Also facebook isn'really a media company. Yes they have some news stories, but that's not what people really go to facebook for, people go there to keep updated on their friends, which doesn't seem like such a threat.
Yes internet and social media addiction is bad and dangerous, but that is something that would exist with or without facebook and can be addressed in other ways besides regulating facbeook