Twitter heavily uses rails, as I'm sure many other big sites do.
The best bet is to pick something that is used by enough large organizations that there is a good bet that if a major contributor disappeared there will be others able to take their place.
Also with open source systems there are generally more discrete parts that can in theory be interchanged with others.
If microsoft did something drastic like announce the end of the Windows Server line (or even just IIS) for example then your entire MVC.Net/SQL Server stack for your app is now defunct.
If Linux was to become no more if would be significantly easier to move your app and it's supporting ecosystem to BSD.
The best bet is to pick something that is used by enough large organizations that there is a good bet that if a major contributor disappeared there will be others able to take their place.
Also with open source systems there are generally more discrete parts that can in theory be interchanged with others.
If microsoft did something drastic like announce the end of the Windows Server line (or even just IIS) for example then your entire MVC.Net/SQL Server stack for your app is now defunct.
If Linux was to become no more if would be significantly easier to move your app and it's supporting ecosystem to BSD.