How do ADS-B and ADS-C get used on transoceanic flights? I have three questions regarding ADS-B and ADS-C. (Hopefully you don't feel my questions are not related.) 

(1) What's the scope of availability of ADS-B? Let's say an airliner equipped with all the necessary equipment for ADS-B in the US flies over to Tokyo. When or where does ADS-B stop working? 

(2) I read from this post that ADS-C is used in some oceanic environments including the Pacific between certain flight levels. Roughly what percentage of transoceanic flights currently take advantage of the ADS-C system?

(3) I assume ADS-C is not a common transoceanic surveillance system, if it's true ADS-C is only used above certain flight levels. If so, what are alternatives to ADS-C? What other (surveillance) systems are being used to follow transoceanic flights? Self-reporting? GNSS? Or what?
1) ADS-B will stop working when the transponder is switched off at the gate after arrival in Tokyo. Airliners use the 1090 MHz Extended Squitter variety of ADS-B which is the worldwide standard.

2) I don't have hard numbers but I guess it is approximately 90 percent of the flights. At most economical cruise flight levels, ADS-C is mandatory.

3) ADS-C is a common system. For aircraft not equipped, position reporting of HF is the alternative. And soon space based ADS-B will be available as well.