Attacks against public instant messaging networks soared over 800 percent in December 2005, compared to the same month last year, a security company announced Tuesday.

According to IMlogic's Threat Center, December 2005's instant message exploits jumped 826 percent over December, 2004, just the latest proof of the expanding threat facing IM users throughout the year.

December, however, was slightly off the previous two months. The year's last month saw 241 new threats, said IMlogic, down from the 307 in November and the 294 in October. Combined, the three months showed a 13 percent increase in IM threats over the third quarter of 2005.

One of the December attacks cited by IMlogic was the "Santa" worm, which posed as a come-on to a Santa Web site and hit users of all the major IM networks, including AOL, MSN, Yahoo, and ICQ.

IM attacks not only continue to grow in number, but also keep gaining in sophistication, said IMlogic chief technology officer Jon Sakoda. "The growth in the level of sophistication and the advanced nature of the malware payloads such as rootkits and information gathering agents sends a signal to IM users that IM security is no longer a 'nice to have' technology," Sakoda said in a statement. "This rise in complexity and destructive nature of the threats requires organizations and individuals to put in place more adequate protections."

MSN was the most heavily-hit IM network in December, added IMlogic, and accounted for 48 percent of the total threats launched. America Online's AIM, meanwhile, tallied 41 percent, while Yahoo's instant messaging network came in a very distant third, with 11 percent.

IMlogic has posted year-end results of its IM threat tracking efforts, including a graph showing the dramatic rise in attacks during 2005, on its Web site.