Le Monde's report states that French external intelligence agency the Directorate-General for External Security (DGSE) has been intercepting metadata from phone calls, emails, and internet activity from domestic services, as well as between France and other countries.
The information was captured and stored in a database in servers that a total of six French intelligence agencies and Paris' prefectural police have access to. The other agencies are the Directorate of Military Intelligence (DRM), the Directorate of Protection and Security of Defense (DPSD), the Central Directorate of Internal Security (DCRI), the Directorate of National Intelligence and Customs Investigations (DNRED), and the country's anti-fraud division, Tracfin.
It is believed that the data is compressed and stored in a datacentre spanning three floors in DGSE's basement. Le Monde has linked the PRISM-eque system to DGSE's supercomputer, which its technical director Bernard Barbier has previously spoken about.
According to Le Monde, the supercomputer is able to process tens of millions of gigabytes of data, and generates enough thermal energy to heat DGSE's buildings.
Le Monde named Google, Facebook, Twitter, Microsoft, Apple, and Yahoo as a few of the sites that French intelligence services are tapping as part of their signals intelligence operations. The paper's report further claims that the metadata is being used to determine the relationships and connections that people have with others to help provide intelligence as to whether it would be appropriate to conduct more overt forms of surveillance, such as phone tapping, presumably for the actual content.
As for the legality of such a system, intercepts are meant to require the authorisation of the prime minister or only after the French National Consultative Commission for Security Interceptions recommends such a course of action. An unnamed source reported to Le Monde, however, that due to the "legal vagueness surrounding metadata", intelligence agencies have been able to store information without technically breaking the law.