You could change your user names to ones you have never used before. Basically start new social media accounts she is unaware of. Delete your OKC and start over for example. FB is tricky as I assume you don't want to delete and restart that. I wonder if blocking the friends she is using to check up on you would help? Not a great option I know and may not work anyway given how difficult it is to truly control information on FB. You may want to consider leaving it entirely.
Maybe some of the more techy members have some stalker foiling ideas?
I also agree with a traceable form of letter (email, certified letter - something not on a social media site) to tell her to politely and firmly stop checking up on you online. I say polite because - gods forbid - if this escalates you do NOT want to have even a whiff of threatening words from you to create a 'he said, she said' situation. Document everything - if this gets worse and hopefully it won't - she's just morbidly curious or something - you will have the information at hand.
I hope this turns out to be a short term annoyance. Good luck.