Hi Brenda:  This can be a tough problem, but great that you know you need to change how you are using your time!  Think about how you access Facebook.  Then put that device (or devices) somewhere far away from where you study in a difficult to reach spot, maybe even somewhere you'd need help getting to.  Then set aside one or two times a day for maybe 30 minutes each time when you will give yourself time to catch up with Facebook.  The rest of the time is Facebook-free!  If you access Facebook on a computer and need that computer for homework, you could take Facebook off your homework computer and put it on another device, following the rest of the directions above.  You might also talk with your parents about some incentive for bringing up your grades.  And, you might need to let your friends know you won't be checking FB 24/7.
I can't emphasize enough how great it is that you want to be proactive about this! Good luck!

Jeanne H. Brockmyer, Ph. D.
education.com expert clinical psychologist