Someone please explain the difference between this a FB's Platform. My guess - Prototypes = useful extensions of FB and Apps = fun & games?
Prototypes is trying out possible new core features. Apps are non-essential functionality.
Platform wasn't sold that way initially, in fact they bragged about how something as core as Photos or Gifts could be replaced by a superior version created by 3rd party developers.
I found the difference now - it's in the description of the main prototypes page: "Prototypes are product experiments built by Facebook engineers".