There is still too much credit to Android/iOS for this design paradigm. Lets nip this in the bud.
The FB app navigation paradigm has NOTHING to do with iOS or Android. Android and iOS both initially used icons, toolbars and tabs (TouchWiz still does).
The new FB app with its button and sidebar was a Path app innovation if I remember correctly. Google too innovated a fair bit in the new age app design (though unrelated to Android, they were doing it for their own apps which were complex and not for consistency with Android) and FB quickly proliferated it. The sidebar is an app UI innovation that came from grassroots developers for apps that have a LOT of features and which would otherwise have very ungainly navigation UIs.
So what you are seeing in this FB app is a Metrofied version of FB's own paradigm. Not iOS's, Not Android's. As everyone can see, this app has no tabs, no icons as such at the bottom. If anything, it bears a remarkable resemblance in concept to the Touch/Mobile Facebook website.
Facebook brought this design TO Android and iOS and now it is coming to WP. So this is FB coming to WP. Not MS pulling in Android/iOS design to WP.
When Instagram finally comes to WP, will we complain that an Android/iOS "app" is invading WP? No Instagram merely brought their app to the various platforms in order of popularity.
At the risk of sounding like a broken record, this isn't iOS/Android design. This is FB design, the design FB would take it everywhere it goes. Since MS made the app, its been Metrofied to fit better with the rest of the UI, all of which is a great thing
Finally, to people still on the fence, I'd like to pose a simple question. If you actually try to think the navigation of a complex app through, you will realise no "heavy" or "complex" app will embrace Metro in all its Pivoty, Panoramy, Large Texty goodness. Pivots serve the same purpose in WP as tabs did in old Android and the bottom toolbar buttons do in iOS, they move you from one category or area to the next.
Now try and segment the various parts of FB into pivots and panoramas, start counting them. FB is so heavy and so feature rich that you would have almost 20 adjacent pivots and side scrolling hell if you did not want to combine pivots with toolbar buttons and categories embedded in menus (Which is an ugly hack some heavy metro apps have been using). The side scrolling pivots and panoramas work great when there are small number of logical segments but they cannot replace the FB or Gmail Sidebar (Similary the *default* MS made WP email app uses pivots for the various labels WITHIN Inbox but does not do so for folders, it chooses a menu list instead. There is a reason why the previous FB app design needed a drastic changed, there was no way all those things could be crammed into the traditional old style Metro paradigm
8 of 10 - Understanding Pivots and Panoramic view controls
Like a Boss! Understanding the Difference between Panorama and Pivot in Windows Phone - Canadian Mobile Developers' Blog - Site Home - MSDN Blogs
Pano versus Pivot: Windows Phone Design Days content available
Windows Phone 7 - Part #5: Panorama and Pivot controls
Blink's Guide to Metro Design | Windows Phone 7 (This is excellent for non developers too)
Links like those meant for developers actually show that Metro is not actually evolving either, all this was planned from the very beginning. In that sense, there has been no 180 or 90 turn. They clearly say when you have more pivots than 6-7, you need to split it into a list view first. Which is sort of what the FB sidebar is in a FB/Metro compromise.
If I was to suggest here that Metro has deficiencies and cannot handle all design cases, I'd probably get labelled a troll but if you trawl through MS's own documentation for developers. They are pretty self-aware about the strengths and limits of Metro design and make affordances everywhere for its weaknesses.
WP/Metro are very new and very exciting but sometimes the radically different navigational architecture really leaves developers scratching their heads with complex apps. I for one, cannot even begin to imagine how a "true" Metro FB app would function whilst maintaining any sort of semblance with the actual FB mobile properties such as m.facebook.com and touch.facebook.com.
I felt gutted when I read 241 comments (at that moment) on WPCentral news that Daniel wrote about new Facebook beta. I was deeply saddened to say the least after seeing the trend in all those comments. The number of woooots and yays for the new design was unbelievable. I'm one of those rare ones who still remember Feb 2010 announcement and a WP without copy paste, still remember what going from big dead space to no dead space at all and death of that tiny arrow. In short, in less dramatic way - I've been around since Metro was put on my palm, indeed in days of WM6.5!
The current beta app according to me is a touch.facebook.com with Metro-ish looking flat and no-chrome with our fonts. Something that resembles iPhone and Android app. Even though the current non-beta app has numerous bugs, the beauty of Metro was wonderful! The space on top that some people say "wasted space" wasn't any more wasted than "People" in People's Hub! But everything in a panorama was how Metro was meant to be (anyone remember Joe's first speech?). It's gone.
It felt like Microsoft itself retracted from the whole Metro - a baby that was so precious that they overhauled Windows with it, they changed Microsoft with it and they couldn't make a Facebook app with it?
In a nutshell, anyone else feels like if this continues, no third party developer might bother with that panoramic, huge typography of metro and just start delivering apps without chrome, flat boxes and vibrant colours only? Will that kill Metro in a couple of years, completely?
Are we bored of it already? I know I'm not!