Is Facebook a necessary tool for a student?
Is there any explicit advantage I would get by joining the course's Facebook page


If you want to interact with your classmates on Facebook before starting the course, then joining the page would enable this.


  or equivalently, is there any disadvantage of not joining?


If you don't feel a need to do the above, then no. There is no reason to expect this to be any kind of handicap. You will have plenty of chances to interact with your classmates face to face, when the course starts.

Facebook is a useful marketing tool for universities, which is why they invited you to join the page. If you personally don't want to interact with others on Facebook (for very understandable reasons), then there is no need to join the page.