“That thing is working better than we could have ever hoped.” Dick Costolo, CEO of Twitter, claims about their advertising efforts.  And the turnover doesn't lie.  In response to a Mashable article.

However, looking at this from an prospective advertiser's perspective, this could be the single biggest waste of money for you and your company.  Taking the stance of a Twitter user, not much interpersonal communication occurs (that's for Facebook), so the network becomes much more of a content consumption and sharing device.  For this subconcious link and urge to occur, the tweet has to be relevant and completely without selling enticement (this is why companies taking advantage of hashtags to market don't work, whereas those creating discussion and sharing content do).  With this in mind, it's obvious to see a contradiction here.  Companies have tried but let's be honest: they're pointless.