You are a Principal System Architect. Design a sharding strategy for a global social media application that handles 100 million writes per second. Address the following points in your response: 1. Choice of sharding key and justification. 2. Handling of 'hotspots' where specific keys receive disproportionate traffic. 3. Resharding strategy for future growth. 4. Geo-partitioning for data sovereignty laws (GDPR/CCPA). Provide a detailed technical explanation.