IM providers routinely archive conversations and traffic logs for periods of time. Depending on your local laws, they may even be required to do so.
Law enforcement (LE) usually "taps" your IM conversations by requisitioning chat logs (what you said) and traffic logs (to whom you said it to) from the IM provider. Depending on your jurisdiction, the legal burden of proof to get a warrant from a judge might be less or more (or non-existent) for each type of log. Check your local laws. Here in the U.S.A., the administration is trying to make traffic logs of all types completely exempt from warrant requirements.
Bottom line is: if you want to keep the
content of your IM conversations secret from LE "wiretaps" all parties must use encryption such as
OTR. If you want to hide the
fact that you are chatting with someone, you need to use something like
TorChat.