It would appear that the time has come for the FAA or NTSB to take over the investigation of this incident as the Country of the aircraft manufacturer Boeing until the aircraft is found.
If the ACARS pinging stopped at 0811 local time (00.11 UTC.) it would be interesting to check the mobile phone records of all the passengers as some will inevitably have left them turned on. If that is the case it should be possible to track those phones when they ping to establish contact with a ground base station if the aircraft flew over land or by intel. satellites.
The next question is did any flight deck crews flying the suspect routes pick up an aircraft without their transponder turned on? Routing scanning of the radar for weather frequently brings up aircraft returns.