Messages are sent in real-time and responses are instantaneous.
Emotions cannot be expressed as well as they can be when actually talking to a person.
Proper conversations can be held with another person without running up a large bill.
People can send viruses through files sent on instant messaging servers.
Files/pictures can be sent in instant messaging conversations.
Instant messaging systems are often used for gossiping and rumour spreading.
It is similar to talking face-to-face to a person.
It is free to use and quick to set up.
Instant messaging systems offer a safer environment than chat rooms.
You can monitor who is allowed to contact you at any given time and people can only 'add' you if they know your exact hotmail or IM address
It is possible to talk to many people at once.
Work can be done in groups, and (if still in school/higher education) the instant messaging system allows people to ask for help before going to their teacher.
Instant messaging systems allows people to talk to each other from different countries, allowing friends to keep in touch if one of them moves away.