I recently ran across a remarkable item that crystallized some of my thinking on the explosion in digital communication that we’re all living through.

Rachel Levy, a marketing consultant in Boston, decided to list all the various applications that help manage the flow of information on Twitter.

Levy lists 10 apps that she uses regularly and 24 more that she uses occasionally. She also lists 62 apps that she doesn’t use– but which are taking up space in her brain just because she’s aware of them.

Now, let’s put Twitter in perspective. It’s got about 10 million users. According to a recent Harvard Business School study, half of the people on Twitter never post a single tweet. And of those who do, 10 percent of the tweeters account for 90 percent of the information on Twitter.

So, Rachel Levy mastered 34 software apps and learned about 62 more– simply so she could communicate with the 500,000 avid users who generate the bulk of Twitter traffic.

I’m not dismissing Twitter. Those 500,000 people are overwhelmingly connectors and influencers. I recently attended a seminar where a number of members of the traditional media said they use Twitter and consider it an important way of getting story ideas and information.

A super-user on Twitter who connects with your message can potentially spread it to hundreds of thousands of people, virtually instantly and at very low cost. So, understand me: This column is not an attack on Twitter.

But it does point to one of the key challenges everyone in marketing faces now and will continue to face in the foreseeable future: how to stay abreast of the unprecedented speed of change in communication.

It seems that every day, in casual conversation or business meetings, I hear a reference to a new software program or information service I’d never encountered before: Envivo. Wordle. Mailana. Grou.ps. Glimpse. Seesmic.

Geeks are cranking away on their computers in basements across the world, hundreds of thousands strong, unleashing their creativity in a way that wasn’t possible until this decade.

Many of these programs and applications will be useful in getting our clients’ messages out to the right people. Many of them will make us more effective in our jobs.

And many of them could be just as great, but we’ll never know– because we’ll miss discovering them among the hundreds of apps that spring to life every day.

Inevitably, many of today’s popular applications will be superseded by others that offer improvements. Twitter has only been around for two years, and for at least half that time, users have been openly wondering when “the next Twitter” will appear.

What does this mean for marketers? It means that we’ve got to work harder than ever to stay informed about all the ways in which we can communicate on behalf of our clients.

It means our clients will rely more and more on our expertise. If it’s challenging for us– we who are immersed in this every day– to stay abreast of changing communication options, how much more difficult is it for our clients, who devote only a fraction of their time to it?

And I’d also argue that we’re entering a phase that carries Marshall McLuhan to extremes. When McLuhan coined his famous phrase, “the medium is the message,” he was primarily responding to the impact of the newest communication technology of his day: television.

With the rise of digital communication, never has McLuhan’s idea rung so true. With Twitter, with Facebook, with dozens of other channels, the message you’re sending isn’t as important as the fact that you’re sending it. If you’re not there, you have no message.

Rachel Levy is there. But can she – and we — learn 96 apps every time a new channel appears? Stay tuned.