Some people seem to think that the Internet is some magical medium that's somehow different from real life. It's not. If you wouldn't ask someone to do it in an offline setting, don't ask them to do it in an online setting.

Would you ask your employee to go door-to-door to all their friends houses and knock on the door and give them a sales pitch? No? Then you shouldn't ask them to shill for you on Facebook.

If you want to advertise on Facebook, there's already a way to do that.