Facebook has changed its policy regarding how you can filter who sends you messages. Facebook is going to make money off people who want to spam you.
If people pay Facebook to send you spam, Facebook will route it to your inbox.
Here’s a quote from Facebook’s policies regarding messages:
“Whichever filter you choose, you may sometimes get messages in your inbox from people you’re not connected to. . . . 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.
If I start getting a lot of spam, I will simply leave Facebook. My friends know how to reach me via email.