I think what happens is they end up segregating their product out into different development lifecycles. It is fully possible to serve a mobile site on the regu

I think what happens is they end up segregating their product out into different development lifecycles. It is fully possible to serve a mobile site on the regular domain depending on the device type. If you're developing for more recent browsers then it should definitely not be necessary. Companies like Facebook, Twitter, Instagram, etc do this to try to capture other users that may have really old devices.