No one is going to switch to a new social network that none of their friends are on unless that new network offers some valuable feature that the original fundamentally lacks. Just look at Orkut: it was a perfect facebook clone with the backing of goolge that became wildly popular in places where facebook hadn't yet become predominant, like Brazil, but it couldn't topple facebook in established markets because no one had friends already on orkut. Google tried and failed to launch many social networks, their mistake every time was that they tried to move on facebook without a sizeable userbase.
Facebook has worked tirelessly to integrate every valuable innovation in social networking very rapidly. They've added hashtags, newsfeeds, video sharing, video chat, etc. No new network has had both a large userbase and a valuable new feature at the same time, and thus no network has been in a position to take on facebook as a general social network.
Presumably Facebook will remain king until either it loses its vigilance (which seems very unlikely so long as Zuckerberg is at the helm) or a valuable new social networking feature is developed that Facebook fundamentally can't integrate. For this latter case, there are in turn two possibilities: Facebook can't integrate a feature because it is too technically challenging (which seems unimaginable given the resources facebook devotes to its engineering and acquisitions) or the feature is just incompatible with the rest of facebook's user experience (in which case this new social network won't resemble facebook much at all). Thus I would posit that, while facebook is not guaranteed to always be on top, there will never be a new facebook that replaces it. Facebook is the Tyrannosaurus Rex of friendster-like social networks.