facebook messaging question!!!?

[A Guy]

I'm looking through my old message/conversation on facebook. When I click on "messages" on the left hand side, I can see a list of people that i've talked to. For most of them, I can see their pictures and profiles, and if I click on the conversation I can "reply back". For some of them and I just noticed that, there's no pictures and just their name (especially one friend), and if i try to send them a message, it says "You cannot reply to this conversation". Why is that?

Also for others, I noticed that there is no picture, and just says FACEBOOK USER. Why does this happen? I dno if i've been block or something.

I even made another facebook profile to check out if that "one friend" had actually blocked me and I could not find him.