I find your post, and some of the replies, incredible. Her marriage of 18 years has ended and quite obviously the unusually distant behaviour is a result of that devastating event.
At the moment, she may be struggling to brush her teeth, never mind checking in with friends. All of her energies will be centred on getting through the day, supporting her children and doing essential, heartbreaking tasks like seeing her solicitor, putting her home on the market or applying for new jobs.
When my marriage ended I didn't answer phone calls or the door. My dog wasn't walked for two months. Basically, I didn't want to face anyone because I didn't want to talk about it.
Looking back on it, I had some fantastic friends. One friend text me lovely, supportive messages every couple of days, prefacing each one with your don't need to reply to this'. One friend rang the doorbell at least once a week. I didn't answer but she left little things on the step - a card, a bunch of flowers, a pizza once.
When I was ready, I resurfaced and my god I appreciated those friends.
OP, you obviously care about your friend but your response to this devastating life event has been less than one would expect from a best friend. I can't believe you messaged asking her what you had done wrong. Any distance now will be because she has gravitated towards those friends who supported her better.
And social media means nothing. She could be spending entire weekends on the sofa in her pyjamas, with nothing to do except fb. It allows her to interact with the world without any real emotional connection.