Small differences in cost can lead to big differences in outcomes. I believe the average IM message is several times shorter than the average "back and forth" email, but this is well-known. The attention given to the other person is also more focal and more immediate. There is less ambiguity whether, and when, a message was received.
Oddly I wonder whether IM is less prone to saying something really stupid. It will encourage more minor blunders in the rapid crossfire, like jokes which are not funny. But the very rapid back and forth discourages truly major idiocies, which require a bit more contemplation.