[kanamekun]

The Apple app store rankings are public though... so couldn't anyone have seen that the app was doing well, and cloned it?

Beyond raw resources, Facebook's big advantages seem to be its well-known brand and its ability to incubate new properties. That brand won't always help though - in some cases, a focused brand will have an advantage. And a viral property could negate Facebook's distribution advantages - although perhaps that's less likely given how they've cut back on news feed distribution.

[leemhoffman]

My suspicion is that App Store rank doesn't necessarily translate into highly used app. Many of the highly ranked apps get there by buying downloads. There definitely is a ton of open data on app success out there, though spotting an app with an organically fast growth rate and high DAU is probably much harder without access to quality data.