The Twitter experiment on Madison Avenue just got more serious.

While marketers are dabbling in Twitter's real-time stream of 140-character missives, and some brands have built teams to interact on Twitter in real-time, investment of ad dollars has been relatively small.

Until now. On Monday, Twitter announced that it had completed multi-year deal with Starcom MediaVest Group, a unit of Publicis, the world's third-largest ad holding company, which represents marketers like Procter & Gamble, Walmart and Coca-Cola, worth hundreds of millions of dollars.

While details of the deal, first reported by the Financial Times newspaper, are vague, the implications are clear: it's both a big vote of confidence in Twitter and a sign of how seriously marketers are now taking platform.

"Over the past few years, Twitter has peen perceived as being in the 'experimental' budget, but SMG clearly doesn't think Twitter is experimental for them anymore," said Debra Aho Williamson, analyst at eMarketer. "They believe Twitter is a vehicle they need to be a part of on an ongoing basis."

I'm not quite sure how they're justifying ROI on ad dollars spent.

I'm fairly sure Twitter does not deliver click throughs like Google and Facebook.

So what are advertisers buying? Brand impressions?