Tweet Tweet Tweet...Metrics?

Lots of talk about Twitter for marketers these days. Is there a return on investment model for marketers using Twitter? Haven't seen it yet. As a tactic, it can work well as a tool to engage in the conversation with consumers about your brand and, more specifically, about issues relevant to your brand and your consumers. But is it any more measurable than pr, advertising, or other tactics? Is direct response for link activity really the way to measure the impact of tweeting acivity?

I love the innovation of Twitter as a new form of communication. As a marketer, it's as interesting as the innovation of email marketing, followed later by IM, as tools for digital communication. If you have a developed a useful metric (not platitudes, actual metrics) for measuring the marketing benefit of using twitter for brand building, I'd love to hear from you.