I think caller is getting names off of Facebook. I socialize with only a small group of friends. Caller says they are a (friends name) or ask if I know where a friend is. I never answer #s I don't know so I just get messages.
If I were you, I would set Preferences on my Facebook Account.
And I would warn everybody on my Friends list to who has that person on Friends list, that they remove his name. Then, this first, I would block his name specifically and change password on my Facebook Account.
That's what I did in the past, and since, never had similar situation.