I took time off of FB, but I'm back on, though my usage has been severely curtailed. I also found a browser plugin that allows me to filter out all political stuff that drove me off FB. I hardly post on there but I found it useful to keep in contact with friends.
I have a LinkedIn account but I can't tell you the last time I logged in (read years).
I find LinkedIn useful professionally: Most of my 'connections' are international colleagues, - quite a number of them individuals with whom I have worked in various capacities abroad.
We loosely keep in touch, and it is an excellent way of getting a 'heads-up' about a new country, or organisation. Someone with whom you worked - and got on well with - in a different country will usually give good advice - or can advise on some of the organisations you may be seeking positions with.
I have written to former colleagues - and some have written to me - asking advice and suggestions.
For the other corporate nonsense, I simply ignore.
FB, I don't "get" at any level, and don't have an account.
Same story few years ago. That person was a friend of about 10 of my FB friends. I asked them who that is. Nobody knew!
Well, as I suggested a few posts ago, ask your friends who have been befriended by - or have sought to befriend - this person whether they know her. If they don't, well, then, you have a decision to make
My recommendation would be to let it go, especially if your friends say that they do not know this individual personally.