>The new disruptor will appear that will initiate a huge shift in users.
Kind of. But it shouldn't be a mental model where something like Facebook, but better, comes along. Rather, you should expect something that fulfills the same emotional, practical need or occupies time that makes people use Facebook less, but fulfills a need that better clicks with what people want.
As an analogy, consider how Apple and Microsoft flipped places. It's not that Mac OS was like Windows but better and supplanted them. Rather, smartphones changed the game in such a way that Microsoft's core strengths didn't seem so strong anymore. Apple was able to innovate in such a way because of their DNA as a design focused product company, but those strengths didn't get to shine until the paradigm shifted to favor cohesive product experiences rather than versatile software platforms.
So the thing that supplants Facebook won't look like Facebook, it'll be some better way to keep in touch with people from previous walks of life, get in arguments with your distant family members, and share photos. And it won't happen all at once either. It'll probably sit as some obscure niche for a while as it slowly refines itself and evolves.