All the information a well-informed terrorist needs for a shoulder-fired FIM-92 Stinger.
In the beginning the info for the flights here in the US was delayed by 5 min. But later on that went away. The info coming down from the planes' transponders is not encrypted in any way, so anybody can have access to it with a small receiver. To be honest, if a terrorist wants to blow up a plane, they can do it without flightradar24.com.
Flightradar24 began in 2006 and planes haven't been blown up with the help of this page. There are other pages just like it like flightaware, etc. Can the terrorists also use it?As anybody can use it, so can the terrorists. Just like they also use the telephone or internet network.
Flightradar24 blocks some flight information from display though for security and privacy purposes. It mostly involves the flights of the royal families or heads of states.