Friends disappearing on sidebar?

[Undead Tyrant]

I logged into Facebook a while ago and noticed that on the sidebar, a couple of my friends have disappeared. Their account wasn't deleted because their account is still there, nor did they unfriend me because I still have them on my Friends List. I searched them in the search box under the sidebar and they did show up, but they're not on the actual sidebar. I logged out and logged back in and they're still not there. Is it some type of glitch that will fix itself, or is it something else?
I looked at the advanced settings and I never messed with it. So hopefully it's a glitch that will fix itself.

[.]

Maybe you hid yourself from chat for those particular friends, you can do that by clicking on advanced settings and choose to appear on chat for certain friends. But more than likely you would know if you did that so it might just be a glitch