If facebook does eventually begin to die, it won't go down quietly. But to be honest, I don't see facebook dieing out any time in the future and being completely replaced. But, facebook losing members is a more realistic possibility. Eventually, the design team at facebook will make a drastic change that will not be liked by a large group of people and they will leave. If this happens enough, facebook may be dethroned by another social media website like Google+ as the biggest social media network.
At some point they are definitely likely to make a mistake. However, do you think that Facebook is already starting to fail as a business and that is the reason as to why Zuckerberg sold some of his shares? Furthermore, the share price of Facebook seems to have constantly steadily drop since shares were released for them!