A few points caught my eye:
- Confirmation that MySQL scales just fine with sharding. More details on their sharding implementation here.
With all the buzz around NoSQL I think people are underestimating good old SQL databases like MySQL or PostgreSQL for example.
- Confirmation that Redis is just great.
- Assigning users to Cells helps handle the combinatorial explosion of users x followers x posts.
To draw an analogy with Ethernet networking, this reminds me of how you can segment a LAN using network switches to reduce bandwidth usage and congestion (as some of the traffic will stay within each cell / segment).