5. The method of claim 1 , wherein calculating recommended user-concept scores for the second set of user nodes of the plurality of nodes comprises: calculating, for each user node of the second set of user nodes, a plurality of user-bias vectors B u (u), wherein each user-bias vector B u (u) is associated with a user node u of the second set of user nodes; mapping the plurality of user-bias vectors B u (u) and a plurality of concept-bias vectors B i (i) to a plurality of sub-spaces using random hash functions, wherein each concept-bias vector B i (i) is associated with a concept node i of the plurality of concept nodes; and calculating, for each sub-space, user-concept scores for the user node u of the second set of user nodes associated with the user-bias vector B u (u) mapped to the sub-space, wherein the user-concept scores are equal to B u (u)·B i (i) for the user-bias vector B u (u) and the concept-bias vector B i (i) mapped to the sub-space.
6. The method of claim 5 , wherein mapping the plurality of user-bias vectors B u (u) and a plurality of concept-bias vectors B i (i) to a plurality of sub-spaces using random hash functions comprises using a random projection process to project the plurality of user-bias vectors B u (u) and a plurality of concept-bias vectors B i (i) to the plurality of sub-spaces.