How would you design a platform architecture for a rapidly growing social media company to ensure scalability, reliability, performance, security, and cost-effectiveness?

Let's discuss platform architecture. Imagine you are tasked with designing a platform for a rapidly growing social media company. This platform needs to handle millions of users, support various media types (images, videos, text), and provide real-time updates. Consider the following:

1. Scalability: How would you design the architecture to ensure it can handle increasing user traffic and data volume? Discuss specific technologies and strategies (e.g., load balancing, caching, sharding).
2. Reliability: What measures would you take to ensure the platform remains available and fault-tolerant? Consider redundancy, monitoring, and disaster recovery.
3. Performance: How would you optimize the platform for speed and responsiveness? Address concerns like database performance, content delivery, and API design.
4. Security: What security considerations would you prioritize to protect user data and prevent attacks? Think about authentication, authorization, encryption, and vulnerability management.
5. Cost: How would you balance performance, scalability, and reliability with cost-effectiveness? Discuss trade-offs and potential cost optimization strategies.

Provide a high-level overview of your proposed architecture, including key components, technologies, and design principles. Explain your reasoning behind each choice and how it contributes to the overall goals of the platform. For example, would you choose a microservices architecture or a monolithic architecture? What type of database would you use and why? How would you handle real-time updates and notifications? What CDNs would you use?