A concrete case study appeared today. Ars Technica is giving away accounts for some new Web site if you tweet a particular phrase (i.e. if you pimp the site). People complained and an Ars employee responded:

In this case, it's obvious that using Twitter is a good idea. Twitter users tend to share opinions about things in a default-public way. That means that if this service is good, Twitter is going to be abuzz with that fact, which will drive sales. It's actually fairly brilliant from a marketing PoV. As far as early-adopters go, you're less valuable than a Twitter user because you're unlikely to share your opinion in a high-profile, searchable, measurable way.

There may also be a tie in with the FTC regulation here; at what point does "viral" "social" "buzz" become paid placement?