How are some people still able to send me messages on Facebook...Please read further details?

[Diana]

after I have my privacy settings set to where only friends can send me messages and these people are not my friends, yet they are still able to send me messages. I asked one person how they were able to do it and did they see the little button at the top right hand of my profile page saying send message and he just said "I don't know how I am able to still send you a message". Someone else sent me a message too and they are not on my friends list. I have it private to where only friends are allowed to send me messages and some of these people that still were able to send me a message are not on my friends list. I know that when a person has it private and only friends can send them a message then the little button will and should not appear at the top of their profile page so that way people that are not their friend can not have the option to send them a message unless they friend them and the person accepts it. What is going on? Did they maybe hack my page or something? I don't know.
@Jeniffer Yep, I checked it after they sent me the message to make sure it was set that way and it was.

[Jeniffer]

Are you 100% sure that you set it that way?
The only other thing I can think of is that they've sent you a message before or you sent them a message if you had them as a friend before then they can still message you