British spies hacked into Facebook to steal information that let them track mobile phone SIM cards worldwide
GCHQ was reading the Facebook and email messages of Gemalto employees because it wanted to find which ones were responsible for sending the encryption keys that SIM cards use.
SIM card encryption keys are useful to spy agencies because they break the encryption on phone calls and text messages, allowing spies to intercept communications without a warrant.
Gemalto sends the SIM card keys to phone manufacturers over the internet. What GCHQ did was intercept those transfers, secretly storing hundreds of thousands of encryption keys for SIM cards in case it needed them in the future.
The Intercept reports that GCHQ used its access to private email accounts to then get into Facebook accounts of people working for Gemalto, as well as other telecoms companies. It's not clear how GCHQ got inside the Gemalto employee emails. But by scanning them, it was also able to get into their Facebook accounts. It monitored those accounts for messages indicating that employees have access to encryption keys. From there, all GCHQ had to do was wait for a transfer of SIM card encryption keys and intercept the data.
The encryption keys gave GCHQ access to millions of phones globally, giving spies the ability to secretly monitor voice and data cellular communications from 450 wireless network providers without the approval of telecom companies or foreign governments.