@Tomasz Kubacki: From my expertise with an enterprise level software background: majority of software (including web apps) is developed under Windows, with .NET/Java, Oracle, and deployed on Windows Servers etc. Most projects are done with WPF/C#, some with SL/OOB to simplify deployment and reach on Mac. After professional development with C#, JS web client development (even with a crutch like TypeScript) looks like a we are back in late 90x.
Pure web apps are just fallback solutions or simply are not interesting, because mobile apps are not very interesting for enterprises due to security reasons of mobile devices. Enterprises still don't trust iOS/Android on mobiles. BYOD is a myth in enterprises which care about security.
After Windows 8, most of WPF/SL XAML apps will be downgraded to WinRT/XAML to make tablet-friendly versions.
Maybe FB, Amazon and Co are deployed on linuxes and developing on Macs, I tell what I see - no macs, no linuxes, no Dart is actually seen, thus needed.
What is definitely regretted is that MS tries to abandon such a beautiful technology as Silverlight neither open-sourcing it nor supporting it on WinRT, nor making it cross-platform, embedding in browser like Flash, etc.