It seems that some basic elements take their position quite quickly:
and then others follow to form the final page:
update (Apr 2018)
I've noticed that youtube is also doing this...
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It's called a content placeholder or skeleton screens. This is a great way to focus attention on progress and content being loaded instead of wait times while the whole app is loading.
About Skeleton screens
Apple have incorporated skeleton screens into its iOS Human Interface Guidelines, calling it "launch images." It recommends showing an outline of the initial application screen without text or any graphical elements that may change.
About content placeholder :
Facebook App (and some other mobile app) made this strategy famous, making users think their apps loads faster.