Are you using a certain service? Do not judge it by its cover. Sometimes the medium someone is using for a service will become some kind of personification of the service. Like for some Outlook is equal to email and TweetDeck for Twitter.
Email or Yammer?
For example some weeks ago I was having a conversation with a colleague on Yammer. At a moment in the discussion I referred back to a previous conversation that was related to the subject, she told me that she read that discussion, because she already received my emails of this conversation. At first this was rather surprising for me, especially since I only spoke to her via Yammer and never send her an email before. When I gave it some thought, I realized that you could also Yammer via email (as you by the web interface, SMS, IM and third party tools).
Email will be always there
What I also realized that we never get rid of email (even if we really want to) and that some services will move from service to infrastructure. Twitter and Yammer are both moving (or already there) towards being a piece of infrastructure (a protocol) and people can use these protocols to communicate with one another. People can choose which tools they use to use the protocol. It could even go further (especially with the tools that take care of distributed microblogging) and tools could even make the protocol irrelevant (which should be done, since it is about communication and not about data exchange via a specific protocol). This way someone might read the information you posted to Twitter on his TV while accessing Facebook.
It will be just a matter of time till services and protocols become more and more irrelevant to the end users, they will not even be aware of the fact that they are either using Twitter or Yammer or Facebook, the service itself is hard to differentiate on anything other than the network effect. The medium will matter, since every medium has its value. It should be the ultimate goal of every service: moving from being a service to becoming a piece of infrastructure.