Summary: after graduation, I sent my professor a friend request on Facebook, but didn't have response for a few days. I feel really bad and I really want to apologize.
I just graduated and I have taken several classes in the last few semesters with a certain professor. As time passed, we developed a great relationship where I felt comfortable going to her office hours and asking questions about class and advice about life after graduation. She wrote a recommendation letter for me for grad school and overall she really had a positive impact on me and I made sure to let her know that after the very last class. I asked if we could stay in touch after finals and she agreed. So after graduation, I sent her a friend request on Facebook and it became clear to me after several days that she did not want to do that. I believe she just preferred to stay in touch through email. I feel really bad and I really want to apologize, but I'm not sure how to go about it. Any advice I can get would be appreciated because she was a huge part of my undergrad journey and I really want to make this right if possible.