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Bill Radke: Ford Motor said yesterday it'll build an assembly plant in China to make the next-generation Focus compact car. Tapping the Asian market is one way the company hopes to grow sales. Another is clever marketing. Reporter Alisa Roth tells us for the Fiesta subcompact, Ford wants to harness the power of the people.

Alisa Roth: Would this convince you to buy a Fiesta?

Jill Hanner: I love it. I love that it's just so small. I love the color, especially I like that I chose, it's pink. It's connected to my blue tooth, so I can just talk on the phone.

Ford doesn't pay Jill Hanner to say stuff like that. Or not exactly. She gets to drive the car for six months for free. Gas included. All she and 99 other "Fiesta Agents" have to do is blog, Tweet and post about their vehicles as much as possible.

Zsolt Katona studies online marketing and social networks at UC Berkeley's business school. He says there are three big benefits to Ford's campaign:

Zsolt Katona: The first advantage is the targeting, directly reaching those consumers. The second thing is it generates considerable traditional media attention, like we are talking about now. And this is also a kind of market research tool.

Because like the rest of the world, Ford can read and watch everything the Agents post. And as Hanner as told me when we were driving in her car, Ford can watch her drive, too.

Hanner: There's a tracking system in the car, so they always know where we are. And when we enter in our . . .

Roth: So they're following you around?

Hanner: They are. I try to think that they're sitting at their desk every, you know, two seconds and like . . . "Where's Jill's car?"

In spite of that -- or maybe because of it? -- just about everything the agents say about the cars, online and in person, seems to be relentlessly positive.

Katona says that doesn't matter:

Katona: The logic goes the person can say anything, so it's more credible. You can also say it's more risky, but I think it's more about credibility.

Or credible because it's risky. Somebody could say they hate the car. But an agent could also get drunk and drive into a tree.

Katona says the real risk is that Ford's not telling the truth: that the Agents aren't really free. If that were true and the public found out, that would really undermine the credibility.

In New York, I'm Alisa Roth for Marketplace.