One of the things that people loved about Facebook, when I was doing my research is the ability for asynchronous conversations (conversation where people can answer in their own time, think wall to wall comments, sms and hand written letters). This gave them time to think up their response aka more time to be witty and charming.. The Digital Native generation grew up with asynchronous conversations tools.
So with the introduction of Facebook IM, which is not a asynchronous conversation but rather a synchronous conversation (on the spot think phone calls, instant messenger, face to face) it will be interesting to see the response. My gut feeling is that it will be like the News Feed, a backlash at the start and then people will learn to love it.
The following is my POV on the strengths and weaknesses of the new application.
All your friends are on there, unlike MSN
No longer have to have conversations back and forward on walls
Easy to use, do not have to install software
Screen is hard to read
It is hard to hide yourself like on MSN (re: Busy)
Does not flash like MSN when I have new comments
Goodbye MSN, Hello Facebook IM