While it's a nice thought that 'it just takes a good idea' but that's patently false. Social networks are heavily effected by network effects (what a surprise?). Additionally, while I know I'm preaching to the choir here, but an idea without execution is beyond worthless. Social networks are extra difficult because there isn't much to drive most people off of one unless all their friends also move.
IMHO the problem with many competing social networks (Tsu, Ello, Diaspora, etc) is that they try to be exactly like Facebook. So in practice they become crappy subsets of Facebook. Then nobody has much interest in using them because Facebook is a much better Facebook.
The more interesting competitors tend to pick a narrower use case (say, sending secret pictures) and become really good at it.
I still disagree, I think we've just seen the beginning of what social networks will do in this century. Facebook is barely scratching the surface, and there is ample room for disruption. It is inevitable that new ideas will emerge, and they will be embraced.
You are moving the goal posts here quite a bit. Do I think that facebook will forever be a giant, without question? Of course not. What I disagree with is that all it takes is a good idea.
I believe that the more likely course of action, as others have stated, is facebook will make mistakes which will be capitalized upon. Large organizations gain cruft, slow down, choose the wrong direction. Many things can happen.
