Them term spim covers spam over instant messaging (IM). Spim is perpetuated by bots that harvest IM screen names off of the Internet and simulate a human user by sending spam to the screen names via an instant message. The spim typically contains a link to a Web site that the spimmer is trying to market.
It is also referred to as instant spam or the less-intrusive sounding IM marketing.

Source: http://www.webopedia.com/TERM/S/spim.html