Instant Messaging spam, sometimes termed spim (a portmanteau of spam and IM, short for instant messenger), makes use of instant messaging systems, such as AOL Instant Messenger, ICQ or Windows Live Messenger. Many IM systems offer a user directory, including demographic information that allows an advertiser to gather the information, sign on to the system, and send unsolicited messages. To send instant messages to millions of users requires scriptable software and the recipients’ IM usernames. Spammers have similarly targeted Internet Relay Chat channels, using IRC bots that join channels and bombard them with advertising.
Messenger service spam has lent itself to spammer use in a particularly circular scheme. In many cases, messenger spammers send messages to vulnerable machines consisting of text like “Annoyed by these messages? Visit this site.” The link leads to a Web site where, for a fee, users are told how to disable the Windows messenger service. Though the messenger service is easily disabled for free, the scam works because it creates a perceived need and offers a solution. Often the only “annoying messages” the user receives through Messenger are ads to disable Messenger itself. It is often using a false ID to get money or credit card numbers.
Source : wikipedia