First, let’s start with a song to get us into the mood:
Now, for the serious business. :)
If you say “Hello!” on Yahoo! Messenger and the person is not online, the message gets sent to the person only when that person comes online. You may or you may not be online at that specific time.
On Google Chat, the message gets sent as an email.
On Skype, things are different, though – you can say “Hello!” and the message will only be sent when both of the persons are online.
So, if you want to talk to someone who rarely uses Skype, you can just open its profile, say “Hello!”, and, when both of you are online, the message will be sent.