For brand marketers looking to figure out whether or not their Twitter investments are paying off, metrics are a big challenge.

Arguably the most prominent Twitter metric, followers, is of limited use in practice, particularly since it’s so easy to game. Other metrics, such as retweets, may be slightly more meaningful, but they’re often difficult to connect to the most important business KPIs as well.

Twitter co-founder and board member Evan Williams, however, knows the metric that could be the holy grail for marketers active on the popular social network. What is it? Speaking at a roundtable this week, Williams indicated that “The dream metric is how many people saw your tweet.”

Such a metric, of course, would effectively enable marketers to measure their reach on Twitter. That could be an important step forward for the company as it seeks to keep brands on side given the large number of quiet Twitter users. As BuzzFeed’s John Herman notes, “it’s hard to tell if these non-tweeting users are actually seeing anything you tweet.”

New metrics: courtesy of Twitter’s complicated relationship with developers

If Twitter gains the ability to tell marketers the number of followers who saw their tweets, Twitter would be one step ahead of Facebook, which has also struggled to pin down a reach measurement that is accurate.

The big question: can Twitter pull it off? The answer may be yes thanks in large part to the changes it has made vis-à-vis developers. Because Twitter has moved to control the consumer user experience, largely cutting out developers in the process, in theory it now has the means to collect more data around content consumption.

With that data in hand, Twitter may have the opportunity to make great strides in convincing marketers that their Twitter efforts are working, or providing them with insight that can help increase efficacy — an opportunity that, if fully exploited, may make Twitter’s snub of developers look like a wise if still-costly decision after all.