Of course, now we extend FB to business use, and since Salesforce.com has some clout people will look into that. Chatter looks neat as per its interface, capabilities etc.
On the other hand Microsoft has many features already available in SharePoint, or IBM has them in Notes.
I am not sure if there is a clear leading product, and believe that the success of these platforms derives from their ability to attract a critical mass of users (ok, business users this time, so numbers will not be as large as the millions).
If that is so, the ability to attract cross-company users is critical, and I bet that this will depend on the fact that these platforms expose their APIs, and have a third parts strategy – possibly focused.