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.
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.