The mistake that people always make with employee advocacy initiatives is thinking about this from the company's prospective and thinking that this is about a tool.  No, in both cases.

To motivate, activate and empower employees to take part, you have to think about "why" from the employees side.  We also have to go back to our change management principles and remember "understanding" and "action" are two different things.

Let's not forget that all employees will be different, they will have a varying degree of understanding of social media.  Some will find it natural, some will be scared and some just won't care.  All need to be treated differently.  Some people will come to the project with a sophisticated understanding of social media.  They might run the social media for a sports club the belong to.  Others may just log into Facebook to keep up with the Grandchildren.

For any employee advocacy program, there has to be a grand vision, articulated by the senior management.  As this is a change program, the senior management also need to lead from the front.

Companies also need to understand this is not about stuffing employee social media profiles with corporate content.  The trick is to activate employees to talk in a short form and an authentic way.  The best form is for them to write short blogs on LinkedIn.

Don't drive readers to a corporate blog, this is after all an "unnatural act" your clients are on LinkedIn , keep the content on LinkedIn.

When to appeal to the "why" of the employees rather than the "why" of the corporate you will find that more people will be willing to step forward and try.  The more people that step forward and then start enjoying, the more you will get a ripple effect across the company.