Hi edita. This works just like Facebook. Also keep in mind that, due to the different time zones, some may be at job, classroom or sleeping while you are online.
Try with emails first, agree with your friends on some time when both are available to chatting.
As Ardilla said, some are just beginning and may want to take advantage of emails and your blackboard, a chance for reading/writing at their own pace.
Check the green 'online' light under your friend's pictures, before chatting.