For reference, the database team at Facebook has always been actively blogging, and keeping up their code available on Launchpad. Its worth noting that the implementation there is based on MySQL 5.0.84 and 5.1.
At Twitter, most of everything persistent is stored in MySQL – interest graphs, timelines, user data and those precious tweets themselves! At Facebook, its pretty similar – all user interactions like likes, shares, status updates, requests, etc. are all stored in MySQL ( …[Read more]