1. One or more computer readable storage media embodying computer readable instructions that implement data structures comprising: persistent data structures and in memory data structures, the data structures being configured to represent streams and sub streams associated with social networking data, the persistent data structures comprising: a stream table configured to record history of a plurality of posts and be utilized to initialize one or more in memory data structures; a following table and cache configured to provide a mapping from a user to people that the user follows; and a notification position table and cache configured to contain a global position of a last element in a stream returned as a notification for a particular UserID; the in-memory data structures comprising: a stream vector that is configured to track stream elements; a substream start vector configured to be used to look up an index in the stream vector of a newest element in a substream; and state information associated with stream.