The newly launched ExecTweets, from Federated Media and Microsoft, is being described as one example of how Twitter will make money -- by charging to promote third-party services on Twitter's pages. But ExecTweets, which promises to aggregate tweets by top executives, actually spotlights another challenge: Despite Twitter's growth, not many top executives are using it.

Sure, there are some recognizable names on the list, such as Sun Microsystems' Jonathan Schwartz, Digg's Kevin Rose, eBay's Pierre Omidyar, Facebook's Mark Zuckerberg and more. But in the case of Microsoft, for example, the company is represented not by Steve Ballmer or Ray Ozzie but by startup guru Don Dodge and business insights director Daniel Rasmus.

To be clear, Dodge and Rasmus are putting out lots of interesting content, and they're definitely worth following. (I was already following Dodge on Twitter, and I've read his informative blog for years. I also just signed up to follow Rasmus.) But on a site created for the specific purpose of tracking the tweets of top business executives, some people might be expecting more.

The absence of higher-ranking Microsoft execs is notable, given the company's involvement in the site. But looking at the people picked to represent other companies on the site, it's also clear that many other CEOs haven't yet gotten on board with this whole Twitter thing.

The case of Jeff Bezos shows that there's definitely interest in following top executives on Twitter. As reported earlier by my colleague Eric Engleman, an account apparently started by the Amazon CEO has attracted a respectable following -- nearly 200 people at this point -- despite offering up only one tweet so far: "this is a test."

Of course, Twitter may still be in its infancy, too young to pass judgment. And maybe there are better things for a CEO to do with his or her time. But one important litmus test for Twitter in the future will be whether more top executives start using the service themselves.