Yahoo! Messenger is a chat application that had used by many user all in the world. With Yahoo! Messenger a user can do video chat, group chat, sharing files, and the other stuff to their friends. Also you can join a room then talk to other YM user although they are not in your friend list.
As online chat application, Yahoo! Messenger give their user to control they visibility as online or not (invisible). Many Yahoo! Messenger user use this feature to hide their visibility to all or certain of their friends. With this feature active, the user will still appear offline on other side although they already log in to YM.
There are online web tool that can detect a Yahoo! Messenger account online status. It can detect is it online, offline, or invisible. Here are the lists:
That list above is web based tool to check whatever an Yahoo! id online or not. Simply input Yahoo! Messenger account to the box and then enter. If the web working correctly it will display that account online status and their avatar (profile picture). You can also use this for other YM user that not in your friends list.