If Facebook exists twenty years from now, it will have to have adapted in a big way. No technology stays the same for an indefinite amount of time without undergoing some kind of revision, especially not one so dynamic as a system for the sharing of information.
Do you honestly think web search will be done the same way google does it, twenty years from now?
Facebook will not last forever. As a company, it will grow in both revenue and employees, and it will take on a tremendous amount of inertia in the process. Sooner or later, another insightful pair of start-up founders (just like facebook was once) will think of a better way to do things, and Facebook, being the giant, lumbering bafoon it will have become, won’t be able to respond quickly enough. Either it will buy that small company, or that company will go on to an IPO and defeat Facebook in the battle for users.
It also may be the case that Facebook will sell itself out to Microsoft, in which case it will be immediately destroyed by the limitless incompetence of that company, and will cease to resemble the Facebook we now know and use.
However it happens, the current incarnation will not be the default format of social networking for very much longer into the future.