The fine has been imposed by the UK’s Information Commissioner’s Office. The fine was calculated using the old Data Protection Act 1998 which can levy a maximum penalty of ..500,000. Facebook has been under scrutiny since it was revealed that the personal data of 87 million users was improperly gathered and misused by political consultancy firm Cambridge Analytica. The penalty could have been much larger had it fallen under EU’s General Data Protection Regulation (GDPR), wherein a company could face a maximum fine of 20 million euros or 4% of its annual global revenue, whichever is higher.
Source: https://thehackernews.com/2018/10/facebook-cambridge-analytica.html