Many years ago I created a communications channel matrix. It acted as a ready-reference guide that allowed me to map out messages first and then choose the channel and the medium that was most appropriate. I have used it ever since.
I even had a version that I used for social media. But then the folks over at CMO.com produced a version for social media marketers – the CMO’s guide to the Social Media Landscape. They have recently updated the guide for 2011. Interestingly it still includes Digg which I find next to useless – and have replaced Delicious with Tumblr. But – as with any form of communications – you need to know where your audience is use the media appropriately. Use this as a start, and overlay your own audience metrics and mapping to make this as customised as it needs to be.