What's the best architecture to do Instagram/Twitter kind of Follow system?

Twitter/Instagram these kind of applications allow you to follow others and share messages/photos.

What's the best architecture to do this kind of system? Or s开发者_StackOverflowimple architecture (like traditional relational DB design) to start and later has a upgrade path?

There is some information on how twitter scales here: http://highscalability.com/scaling-twitter-making-twitter-10000-percent-faster

Two of the things that they do are:

• Keep things in memory
• Place data in correct place when it is written, rather that getting it when it is read