My Facebook chat bar shows some weird icons, see here:
What has happened to it actually?
That has happened to me as well. As far as I can tell, the open green circles indicate is a person has a mobile device connected to Facebook, similar to what used to be a little phone icon next to their name. If the person is actually logged into a computer and on Facebook available to chat, the they will have a full green circle next to their name.
That's what I thought to, but right now I have a friend that I'm talking with on Facebook and she has the little hollow circle... Everybody except 2 people have this little circle :(, oh Facebook...
The Green Circles show weather your friend is online and available to chat, if there is no green circle then they are offline or unavailable to chat.
Those icons,if present besides your friend's name can be an indication of their being online.