When you really need to get in touch with users fast, use the wall command to write to all who are logged on.
Reserve the wall command for those times when you need to ask users to log off quickly. For example, you can warn users of unexpected shutdowns:
# wall The system is coming down in ten minutes for unexpected maintenance. Please log off soon in order to save your files. <CTRL-d> #While wall is unmatched for getting urgent information out quickly, some users dislike having their work interrupted by an uninvited message. Many users guard against this by including the command mesg -n in their .profile. This command blocks the output of wall sent by ordinary users. But a user with appropriate authorizations can execute the wall command and override the mesg -n command. See wall(1M) for more information.