I believe the big difference is the virality multiplication factor that Facebook provides. Sure, you can send SMS, but you might have 200 contacts at most. And most of them will not forward your message. So you might reach 1000 people max, if your SMS is really hot. On Facebook, viral posts will get displayed to more and more people that press the like button and spreads to many more people, especially people that do not know the originator personally, and thus he has no reputation to lose.