I suggest you read about the new European General Data Privacy Regulation, which comes into force in May 2018, and requires you to allow users to (a) export and (b) delete all personal data they gave you, including messages, posts, etc. Violation will be punished with a fine of up to 20 Million EUR or 4% your global revenue, whichever is higher. This law will apply to all sites that have EU customers, no matter where the site is located, and will be enforced globally (through banks assisting with account freezes, etc).