15. A non-transitory machine-readable storage medium comprising a program containing a set of instructions for causing a machine to execute procedures for improving data compression efficiency of a social network system, the procedures comprising: receiving an instruction to improve data compression efficiency of social graph data of a social graph, the social graph including multiple vertices representing social network users, some of the social network users are friends in a social network; generating based on the social graph a partitioning tree including multiple bottom-level buckets, the partitioning tree dividing the vertices of the social graph into the bottom-level buckets and ordering the bottom-level buckets such that a social network metric regarding the vertices is optimized; assigning user IDs to the vertices of the social network in a numerical sequence based on the ordering of the bottom-level buckets; and storing the social graph data of the users in storage locations in an order according to the numeral sequence of the assigned user IDs of the vertices, wherein the social graph data of the users are stored in a form of differences between the social graph data of respective users and the social graph data of neighboring users.
15. A non-transitory machine-readable storage medium comprising a program containing a set of instructions for causing a machine to execute procedures for improving data compression efficiency of a social network system, the procedures comprising: receiving an instruction to improve data compression efficiency of social graph data of a social graph, the social graph including multiple vertices representing social network users, some of the social network users are friends in a social network; generating based on the social graph a partitioning tree including multiple bottom-level buckets, the partitioning tree dividing the vertices of the social graph into the bottom-level buckets and ordering the bottom-level buckets such that a social network metric regarding the vertices is optimized; assigning user IDs to the vertices of the social network in a numerical sequence based on the ordering of the bottom-level buckets; and storing the social graph data of the users in storage locations in an order according to the numeral sequence of the assigned user IDs of the vertices, wherein the social graph data of the users are stored in a form of differences between the social graph data of respective users and the social graph data of neighboring users. 17. The non-transitory machine-readable storage medium of claim 15 , further comprising: responding to a social graph data request for a list of requested users by retrieving social graph data of the requested users together from storage locations that are close to each other, wherein the requested users are related users who have been assigned the user IDs that are close to each other.