If an employee does not use social media (they don't have a facebook account, don't use twitter), you can't really require them to use their own social media accounts to promote the company. If they are required to create a new account, it probably won't do much good since it will have only been created just to promote the company and will exist in a very small social network.
If an employee has a social media account and they use it to promote your company, consider that there may be cases where you don't want to have your company associated with their social media account and the image it presents as it may reflect negatively on your company.
If an employee is required by corporate policy to promote their employer through their personal social media account, there's a good chance they may resent it and feel like they're being used as a cheap/free PR tool. This could backfire if they decided to subtly (or not so subtly) sabotage the promotional campaign. Or they might flat out refuse. This could cause bad feelings throughout the office if people are required to do this even if they don't want to.
Providing optional incentives for people might work, but you'd probably first want to make sure the employees are actually interested in this before starting up such a program. Otherwise, don't do it. Hire a proper PR/marketing team.