Facebook friend vanished?

[Ben]

This is the second time now, two people from my friend list have just vanished but they are still on Facebook. Why is this? Could they have blocked me? But they were really good friends and have nothing against me in real life?

[Michelle]

They probably unfriend you

[Just]

They probably unfriended you for facebook-related reasons... do you post controversial statuses, comment a lot on friends' statuses and pictures, or maybe say things that might be embarrassing to them in comments?