Social Applications and Messaging

What would you think a good social application for a business should look like? What features do you think it should include?

Messaging? I'll say that is a basic requirement for anything to be called a social application.

Should that messaging be a stand alone system? If it was you would only know you had a message when you went to that application. I don't think that would be very satisfactory.

The messaging could connect with your email like Facebook does. That way you can choose to respond through your regular email or through the application.

Would you want it to have a standard public API? That would allow third parties to create a variety of services - like TweetDeck for Twitter - so you could have many more choices about how you could be notified about a new message. This would also give you more choices of how to respond.

Are there other options I am missing?

Which ones would you prefer?