The default message filtering in Facebook is set to “Basic”. This means that messages sent to you by those whom you don’t know often show up in your Messages inbox.
Here is how to setup strict filtering so that only messages sent by friends and close acquaintances show up in “Messages” while rest go to “Other” inbox :
Login to Facebook account and select the gear icon, then select “Privacy settings”.
Under the section “Who can contact me”, enable the option “Strict Filtering”.
This will now for most part filter messages and you’ll see those messages (mostly from unknown / not close acquaintances) in “Other” inbox rather then directly in “Message” inbox.