Would've loved it if they had open sourced their real-time graph solution.
I still don't quite know when I should use graph database but I imagine for social networking type of websites it is a must (since a standard RBDMS or NoSQL gets too verbose).
A relational database plus offline processing is usually enough for that, e.g. FB uses MySQL
Graph structure based features like in Friendster are expensive and don't scale well, that's why Friendster failed. And MySpace and FB removed that features (friends of friends of friends) early on.
Good point. We actually use MySQL and Regis for persistency with a helping of memcached for good measure.
The "graph" part is an API view on our data, not the canonical storage.
I still don't quite know when I should use graph database but I imagine for social networking type of websites it is a must (since a standard RBDMS or NoSQL gets too verbose).