Short story: employees who feel engaged and trusted will advocate for you and your products. Long story: there is probably more to it than that.
In Do Your Employees Endorse Your Products? Bill Ives comments on a Forrester report of employee engagement by Matt Brown on the same topic, which was described in his blog back in November: Employee Advocates Emerge From Empowered Workforce. The study and report look at how and whether employees advocate for their company via net promoter score. The interesting finding is that technology employees who use social media tend to be more inclined to promote for the company than those who are restricted or don't use social media. Actually those who are restricted appear to have an overall negative score (they would detract more than they would promote).
I have to wonder if it is just technology employees who fit this statement, or if that is only who they surveyed. Why would anyone who doesn't feel engaged in their work and trusted by their employer be inclined to be positive about the company and their products? While the study in question focused on technology employees, I don't see why these findings wouldn't apply to just about anyone in an organization.
My take on this information and based on my experience (and similar reports that I've seen over the years) is that when people are engaged in their work, they are excited about it. They want to do the work. They are excited about their organziation. And that would naturally lead to people advocating for their organization: attracting people to work with them, and encouraging people to use their products and services.