There's also what I might call the "serendipity factor." Sometimes you discover that an unexpected network is big among your community. With PBS MediaShift and a NY local tech blog, for example, we've seen the occasional spike in traffic from StumbleUpon, a service we had not previously concentrated on. I have even seen some spikes in activity from niche networks, such as AllTop (which if memory serves is run by uber-marketer Guy Kawasaki). It also changes over time, so it's important to pay attention to the flow, and the changes in use of and abilities of the tools. When Google+ really opens up, and when it releases an API, we'll probably see spikes there. Here's a project I'd like to work on and contribute to: A tool that has the intelligence to do a lot of this for you: IE, it knows that Google+ will work best in this instance, Facebook in that, Twitter in another. I just plug in my content or my need, and the tool decides where and how to send it. We're seeing some nascent attempts (SocialFlow, Measured Voice) but none that yet quite get it right and load balance among all the social media and are able to quickly incorporate the unexpected spikes in new networks.
Social Media's Nuanced Differences
In the Social Journalism Educators group on Facebook (which I believe is invite only), @emilybell of Columbia U's Tow Center alludes to a great point: Understanding the differences among social media is a core piece of knowledge, and how use them in a given situation a core skill. I've used Facebook to reach out to certain types (for example, suburban mothers of school-age children for one hyper-local news project), Twitter and LinkedIn for other groups or purposes. Am still learning Google -- yet another one to learn -- which at this point seems biggest among the tech influencers or 'digerati.