Efficient social allocation awards item to the highest valuer in a sponsored search auction. Whom is it optimal for?
The optimality concept used here is usually Pareto-optimality, a.k.a. Pareto-efficiency. 


  Pareto efficiency or Pareto optimality is a state of allocation of resources from which it is impossible to reallocate so as to make any one individual or preference criterion better off without making at least one individual or preference criterion worse off.


So a Pareto-efficient allocation is not necessarily 'optimal for' anyone. It just means that given the current allocation no one can be made better off without cost to others.