A question on Facebook messages?

[Lenny]

Filtering?

You can change your filter preferences right from your inbox.
https://www.facebook.com/help/116215745206219

Basic Filtering
If you select Basic Filtering, you'll see mostly messages from friends and people you may know (ex: friends of friends) in your inbox. Typically, messages from other people will go to your Other folder, and you won't receive a notification about them.

Strict Filtering
If you select Strict Filtering, you'll see mostly messages from friends in your inbox. Some messages from people you want to hear from may go to your Other folder.

Whichever filter you choose, you may sometimes get messages in your inbox from people you're not connected to. Some examples of when these messages may go to your inbox are:
A friend of a friend includes you in a message along with some of your mutual friends
A member of a group you're in messages you or includes you in a message
A friend who isn't on Facebook uses your contact info to send you a message from the Facebook Messenger app
Additionally, someone you're not connected to on Facebook may pay to ensure their message is routed to your inbox instead of your Other folder.