[nine_k]

Fail fast works on the web, where you can redeploy the app with the next page refresh.

It works poorly on mobile, where users are not keen to reinstall an app every few days, and some do not update for months and years, because of lack of space, scarce bandwidth, old hardware, or just neglect.

[munk-a]

It doesn't just apply to the web though - it can even apply to OS design. On the web it's usually (ab)used to leverage users as the metric of whether something is failing, but in theory fail fast is just about learning that something isn't working through any means - whether that be user reports, automated tests or proofs of concept.

Additionally, at least where Facebook is concerned and IIRC, they actually do heavily utilize out-of-app data in their mobile app. There is a good deal of code, but a lot of the UI ends up being tweaked by data that's being served to the client.