I'm not sure that Facebook using Instagram as a way to funnel people back to Facebook is a great strategy. Companies need to be willing to cannibalize themselves or someone will do it for them. I don't know if Zuck had the foresight at the time, but Instagram has obviously been an amazing hedge against apps like Snapchat. People like Instagram and Snapchat partly because they are less formal than Facebook, most users…
I think it is remarkable that this experiment was repeatable.
Zuckerberg has hit upon an amazing strategy in a world where almost no one cares where the money goes: when a platform is dying, buy a new one relatively quietly, ferry customers over to the new platform. Rinse, repeat.