Since the dawn of e-mail, it has been used for every type of person to person communication that someone can imagine.
The thing is e-mail was getting used for so many different purposes that messages were getting lost. People were sending messages to say that they were on the way home from work and it was getting mixed in with everything else.
This is where lightweight messaging services like the Facebook Wall, Twitter, or Pownce. They fill the role that email did but they do it better than email.
The Facebook Wall allows you to say a quick hello. Twitter says what you’re up to. Pownce allows you to send someone a quick link or file. They each only do these simple tasks and they do it better than email does. Plus with the messages siloed out into these different services, you can process them much quicker.
As Jeremiah Owyang points out, because of these lightweight messaging services, e-mail usage may be on the decline. It really doesn’t surprise me.
Oatmeal Stout - Justin Thorp's blog 
I definatly notice instead of sending emails I am putting messages on my friends walls on facebook, sending sms text messages, or sending twitter messages.
I still use email on occasion. It’s the one service that everyone has. In fact, I haven’t signed up for any of the the services you mentioned above, and I really don’t see the need to.
@Keith, I think you’re right. There needs to be a level of ubiquity that the app needs to reach before it becomes useful. A lot of my friends use it, which makes it for me really fun and useful.