The Irish Data Protection Commission has formally begun an investigation into Facebook's recent data breach. It will now decide whether the firm should be fined for failing to prevent hackers from being able to access up to 50 million users' accounts. Earlier this year, the social network picked the regulator to be its "one-stop shop" for oversight of its compliance with EU privacy rules. In theory, the watchdog can fine the US firm up to 4% of its global turnover. Earlier, Facebook had declared that third-party apps and services which let users log in using their accounts had not appeared to have been compromised in the security attack.