DSis wasn't unpopular in school, but she was the sort who had a circle of 4-5 really good friends and was well-liked by the rest of her class. Kind of kept to herself and her small group. I had my best friends, but was much more outgoing......MUCH more outgoing.
She set up a FB account and wanted to keep it small, no big surprise. A cousin send a friend request. We come from a county in which both sides of our family go back 100-150 years. We have squillions of cousins.

After a few decades of marriage, when we go back to visit, DH is STILL meeting cousins he's never met before.

Anyway, DSis thought it best to accept the request since Cousin was always nice enough and you hate to reject a cousin in the South. (FB request, not marriage proposal...

) Next thing you know, a lot of people who must have been on Cousin's friend list start requesting her as a friend or however you phrase it.
She told me she didn't even run with these people in HS, so why on earth would they want to be her friend now?

I think she has just ignored them, rather than reject them. She's too private to let just anyone on her list.
I've never even joined FB. First, I hate them because DH's work used to be tied to them and they kept him overworked.....Yes, it was personal.

Second, I told DSis I'm half afraid that all those people I was "friends" with back in HS would come out of the woodwork and then I'd be in the same pickle as she. Reject, accept or ignore? Not long ago, my boyfriend from when I was 13 tried tracking me down through relatives and freaked them out. (They did not give him my number.) Why on earth did he want to find me decades later? He has to know I'm probably married and I know for a fact he married at some point. Heck, I think he had 6 kids. Even if I was available, I'd run from a man with 6 kids.

And I had a stalker in college. I still look over my shoulder for him. So I'm afraid that if I got on FB and friended too many people, who then friended someone else, that a reasonably motivated person could find me when I didn't want to be found.
Therefore, if I ever joined FB, like my DSis, I'd limit my friends to a small group. Maybe that's it. Maybe over the years, this person has had reason to become private. Pics get posted on facebook and have a way of getting out. That worries some people. It could be all sorts of things. If this individual is nice to you in person, I wouldn't worry about it.