Why Instant Messaging is a bad substitute for being together
The waiting part
When you really miss someone and know that the person occasionally is online, well then you are afraid of running past each others, which means that you spend a lot of time waiting for each others, just to be disappointed with the kind of contact provided by the instant messaging system.
When you are in love, you need a lot of intimacy, something that a IM service cannot provide. When you are at a computer it can be hard not surfing a bit while talking, not chatting to others if they start chatting to you, not checking your mail etc. This means that you are never really alone together, and thus not there 100% for each others. And so you get irritated with each others for almost no reason. When you are seldom at the internet, it becomes even harder not doing other things.
It also proves very hard to transfer feelings and interpret feelings correctly, which is a big part of the intimacy. Emoticons are just not refined enough and you can easily hide behind them, showing other feelings than you have; something that is harder to do with facial expressions.
Webcams help a bit, but you cannot tell if the other person is really looking at the camera or it is covered with another window (browser window, for instance), so it is not such a good solution either.
I saw Instant Messaging as an easy way to gain intimacy and share my daily life with my fiancé now that we are apart for some time. Instead we both realized that the supposely pleasant intimacy tends to get ruined, and that we have to find others way for gaining intimacy.
The Instant Messenger gives you a fake sense of intimacy, and when you realize that things are not the way you assume, you get sad, irritated, and start missing each others even more...