This is the message I am receiving:
Only add people who you know
Please only send friend requests to people who you know personally. If you think you're seeing this by mistake, please let us know
Why am I receiving this message?
Facebook is working to improve their security and algorithms which includes cleaning up current "fake" or "hack" accounts by "people you don't know" and encouraging people not to accept friend requests from these accounts.
I think this is part of a larger marketing campaign for FB to regain security and trust after it was found out that FB was somehow used by a 3rd party to sway elections. I'm not familiar with what happened in any detail though. Here's a recent NPR article about the scandal: https://www.npr.org/2018/04/11/601323233/6-facts-we-know-about-fake-news-in-the-2016-election