It's probably just phishing.
Everytime you click those links, your username and password gets stored. Then a bot logs into your account and sends those messages to other people.
It's happens on everything and isn't just an issue with adium. It could happen on iChat, Skype, AIM, etc.
Is this a virus? Should i change my password?
Not a virus. Change your password and it will stop yourself sending those messages if you've been phished. Just don't click weird links from people (Whether or not you're friends with them or not). Ask them what it is, first. And if they just give automated responses, don't click it.
"Free Offers" is a suspicious one. So is "Is
this picture you!?" (One I almost got struck by once... although, that WAS a virus *I was on my old PC* but thankfully I had FireFox, so I could terminate it easily). If you're on a Mac, you won't get viruses so you're safe. If you know the person most definitely wouldn't have their computer on at that time, then it's someone else (or something else) on their account sending you these malicious messages. To be honest, they only take your passwords for the IM service, so I don't see the point in it... but hey, people are b*rds. (I meant
birds... ummm)