Facebook is vulnerable to the next thing more than many technology firms that have benefited from network effects in the past. If users are given compelling reasons to switch to something else, they can with less trouble and they will.
That compelling reason might be a new social network with better features or an awesome distributed architecture that allows freedom for users and the ability of those users to benefit from new and fantastic things that Facebook's overseers would never let them have and without the things Facebook's users suffer through today. Or it might be a sexier proprietary box to store users' private information. It doesn't mean that I'm not worried about Facebook. I remain deeply worried. It's just not very hard for me to imagine the end.