It was the late 90s, social media platforms like Facebook didn't yet exist, texting was in its infancy, email was very popular, instant messaging was up for...
The bottom of the article links to the history (individual features) of other IM programs from that era as well like ICQ and Yahoo Messenger.
The reason MSN stopped being used was because Microsoft started requiring Microsoft accounts for it to work, and started pushing people towards Skype. Which is why "Microsoft happened". I never really meant to imply that Microsoft bought it or anything, just that they are the reason it eventually died.