How can i keep others to send me messages o facebook?

[Lenny]

You can set your privacy settings to "Strict Filtering"

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