Sometimes some of your friends who appear offline in yahoo messenger may not be actually offline,they may in the 'Invisible' mode.This maybe if they are trying to ignore you or are too busy to talk to anyone.
There is this small trick that you can use to find out what the truth is.
Firstly open your yahoo messenger main window and double click on the name of the person whom you want to check.The chat window will open obviously.
Click IMVironment button, select See all IMVironments, select Yahoo! Tools or Interactive Fun, and click on Doodle.
After loading the Doodle imvironment there can be two possibilities
1.If the user is offline Doodle are will show this "waiting for your friend to load Doodle" continuously .
2. If the user is online (but in invisible mode), after few seconds (it can take up to one minute, depending on your connection speed), you should get a blank page So you know that the user is online.