IM is always running in the background. Facebook requires going to the website and logging in and monitoring it. And being "available" to a huge swath of people. Not to mention, all of my colleagues and co-workers are on IM. I'm not going to reach out to a colleague to ask about a client via facebook chat and I'm not going to communicate with a client via facebook, instead of IM.

In short, Facebook likely contains everyone you've ever met or dealt with in person or online and "friended" that can potentially chat with you -- IM likely contains only the people you actually need to chat with.

"Everyone is there" is becoming an increasingly poor reason to justify selling your soul to a particular social network (as an individual or a business).