Individuals involved in the conversation can save IM sessions as computer files.
Sometimes they do.
If a page, five years ago, was upset by undue attention paid to him or her by a member of Congress, and reported it to no avail, he or she might save the IM sessions to support their claims, in case anything came up later.
If someone were to suggest that this is too much for a teenager to think of, think again. Teens will often hang on to hurtful correspondence, from their peers and from adults, for a long time, because they do. They're used to not being believed by adults, and they know their peers have selective memories, so they want proof of what really went down in case the whole thing blows up. I see it all the time.