Searching for IM accounts, I don't think you can really do.
But (assuming you're using Windows) you may be able to tell what other users are on the network, either through good old Network Neighbourhood, or a commandline 'net view'. If nobody shows up there, I suppose you could try scanning for active IP addresses similar to yours, but that's a bit more effort.
And if you find someone on, you can see if they've got any open shares and copy them a virus greeting text, in case they check -- or if they're using Win2k or XP send them a Windows net message with the 'net send' command.
Of course, if you then find out they're just a clueless (l)user and not another geek, you may have just freaked them out by 'hacking into' their computer. But that's another way to meet people and perhaps earn a reputation (; |