There are a lot of implicit social expectations built into different messaging systems. I have friends who I regularly email, SMS, and IM with. We've never exp

There are a lot of implicit social expectations built into different messaging systems. I have friends who I regularly email, SMS, and IM with. We've never explicitly discussed this, but it's pretty much assumed that sending an SMS means you want a response ASAP, an IM expects a response within a few hours, and an email can wait a day or two. Even though all three are ostensibly messaging systems, there is a social understanding that different messaging mediums have different purposes; simply filtering based on sender doesn't convey enough contextual information to make a meaningful decision.