PLEASE, instead of speculations, and spreading false info, read the info on the page
About flightradar24.com - a site with high-flying plans!
"How does it work?
Flightradar24.com shows live airplane traffic from different parts around the world. The technique to receive flight information from aircraft is called ADS-B. That means the Flightradar24.com can only show information about aircraft equipped with ADS-B transponders. Today about 60% of the passenger aircraft and only a small amount of military and private aircraft have an ADS-B transponder. Flightradar24.com has a network of about 100 ADS-B receivers around the world that receives the information from aircraft with ADS-B and sends this information to a server, and then displays this information on a map on Flightradar24.com. Only aircraft within the coverage area of the 100 receivers are visible."