InstantMessaging is fundamentally the same as e-mail except it provides PersonalPresence? on top of messaging. That is, it tells Alice whether or not Bob is around to receive the message. Thus, when Alice sends a message to Bob, and Bob is around, she has the expectation that Bob reads it immediately, and in theory Bob should then reply it immediately. In practice this is not required, as Bob could spend a lot time before he replies, but the social pressure of Alice knowing Bob is there compels Bob to reply immediately.
In practice, InstantMessaging also adds some online connection between Alice and Bob, either through a central server (e.g. MSN, AIM, ICQ), or through a PeerToPeer. In this way, InstantMessaging also guarantees the message was received, and not 'lost' midway through, further increasing the social pressure to reply as Bob cannot reasonably claim he did not get the message. Further, the UserInterface to reply is usually as simple as writing one line and hitting enter, versus going through the whole process of composing a new e-mail. Also, it collocates all messages in a conversation with one person in a window, rather than pooling all messages in a chronologically sorted list.
InstantMessaging has taken the place of the phone in many circumstances. It is both real-time and transaction-based in the sense that replies usually come right away, just as in InternetRelayChat, but they can also be deferred if work comes up. In that way, it is 'unobtrusive' as it is interruptible. However, given its ease to fill in the crannies between work, it has a tendency to expand to take up otherwise productive hours.
Instant messaging is more of an Wiki:AntiPattern (Wiki:DarkPattern); after all, stateless, context-free messages are a bad way to continue a conversation.
I think, instant messaging is used mostly because it takes fewer social graces than the phone, so if you're introverted, it's easier. At least that's what I see amongst the people who use it, myself included. Phoning compscis is socially frustrating.
Personally, I use it for the same reason I use e-mail. AsynchronousCommunication (related: TransactionBasedCommunity). However, e-mail is superior because it gets through firewalls, includes state much easier, is decentralized and the clients are better. Instant messaging does provide you with knowledge of who is online, but in my case that is always false information (my computer is always online).
One anecdote. One summer I had ICQ running at work and so did my roommate. As this was the first summer of university, most people had gone back home. Of all the people on our contact list, the only people online were myself at home, my roommate at work and myself at work. Despite the fact we both knew the other was just sitting there coding away, we were both (pointlessly) N/A all the time. Sure enough, we'd message each other anyway every so often. It was really stupid.
By the way, look at France and their Minitel/videotex system for one of your examples as well as pen-and-paper letters (which are stateless, though not instant). -- SunirShah
I believe that communication techniques fill a spectrum, running from full Asynchronous with long delays, down to fully synchronous and completely immediate. They also range from completely private (e.g. we can assume a letter in a sealed envelope or an encrypted email is private) to almost completely public (InternetRelayChat?). Different techniques at different points on the scale have different uses. InstantMessaging is mostly synchronous and mostly private - which fills a niche. The same niche is also filled by /msg'ing on InternetRelayChat.
It's an important niche to fill - cell phones are quite popular for just this sort of thing. -- ErikDeBill
Lately, I've been playing with the JabberProject. I've changed my mind. I still don't see much of a reason to use InstantMessaging for private messages or chatting, but I see it being useful for a much larger range of functions. In fact, InstantMessaging might supplant e-mail if the trend keeps up, especially something as amazingly flexible as Jabber. -- SunirShah
Instant messaging is really the thing for teenagers on the Internet. Many parents have expressed concerns about the amount of time their children spend on the Internet in lieu of face-to-face relationships. Many have also expressed concerns for their children's safety and development.
["Teenage life online"] asked the basic questions to determine what is indeed a reality. For anyone who spends much time thinking about the Internet, the answers these kids give may not be too surprising, until you realize that these kids have discovered how the Internet works more or less themselves, while still in the process of developing into adults in the usual manner. At least this might alleviate concerns you might have about teenage use of the Internet.
It is also a good way to communicate when working remotely. I find that a large percentage of the engineers I work with won't bother me with an email, but will send an instant message for what they consider minor things. (They'll also bring these issues up on the phone if I call them.)
As to it being a Wiki:DarkPattern, I agree that its not good for continuing a conversation. Its really great when you can convey the entire context in a single message, and with history functions, reviewing what was said before is not as hard as it was before.
EMail ends up being for heavy thoughts and serious topics. Instant Messaging is better for checking the weather, chatting about movies, and asking quick questions.
Most importantly, while it relatively easy to miscommunicate in this medium, as well as poor for carrying out important conversations, it is an excellent means of leaving a message that will be visible to the person on their return, or alert them at the moment. Coupled with the hateful, but functional AOL QuickBuddy, it can be used to send messages from any Java-equipped box in any remote location. For those who don't like the cost or hassle of being able to be cellphone-available at any moment, instant messaging is a less troublesome though more crude means of communication. It is also the option of choice for internet-related acquaintances with whom exchanging phone numbers or even email addresses might seem risky.