For users at home and at work in August, MSN Messenger led the pack with 28.6 mln unique users, followed by the once-dominant AIM with 27.6 mln, according to Nielsen/NetRatings. Yahoo trailed at 18.1 mln, the research firm said. MSN users spent an hour and a half using IM every time they launched the software. AIM users spent five hours per session, and Yahoo Messenger users typically logged on for about two hours and 50 minutes, Nielsen/NetRatings said.