What is Facebook User in my inbox?

[An. K]

So example my friend name A. but sometimes i see people in my inbox not A instead of "Facebook User", what does that means. does it means someone block me?
I have try to research on the internet but the result i got is not what i'm asking for

[Tamika]

it meens they'v blocked you or you'v blocked them

[Salem Saberhagen]

It means said person either blocked you, or deleted their account.

[Kiz]

Usually it means they've deleted they're facebook account. If they had blocked you theielr name would still be there but it would be black. I deleted a facebook account cause I had 2 and the one I deleted says facebook user on the messages.