I just want to emphasize again that Flux is entirely possible without singletons, and works just as well on the server if you create new instances for every request. Flummox does it, Fluxible does it (at least for stores). It's just a shame Facebook pushed singletons and then everyone followed their lead.
I also think flux's real-world implementation came from the necessity to build React components within pre-built apps, where they simply didn't have the ability to pass down props because they had to create complete separate components.