Why not create a mesh network using the planes themselves. Surely most planes must be in line of sight with several other planes even over the ocean. That way they could broadcast their position among other things.
"It does, however, have an air-to-air capability designed for use in environments like the ocean that allows aircraft to “see” each other—the FAA is now trying out this technique."
Sounds like it could work in much of the US and Europe, but planes are more sparse in other parts of the world. Africa for example has minimal ADS-B coverage and a relatively small number of planes in the air at once: https://flightaware.com/adsb/coverage
ADS-B is only really used currently as a secondary resource for identifying aircraft rather than seeing where aircraft are. The primary way is by using a radar return, and matching this up with the aircraft transponder which reports the aircraft ID and altitude.
At the moment I don't think there are any hard requirements on having ADS-B transmission equipment on aircraft, although most commercial airliners already have it (which is what FlightAware and such usually use - at least outside the US). It will become a requirement over the next few years however for most civilian aircraft.