The Collaborative International Dictionary of English v.0.48:

Unsociability \Un*so`cia*bil"i*ty\, n.
   The quality or state of being unsociable; unsociableness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unsociability
    n 1: an unsociable disposition; avoiding friendship or
         companionship [syn: unsociability, unsociableness]
         [ant: sociability, sociableness]