I went to a workshop on the treatment of social anxiety disorder with David Clark in July. It was very helpful. I've listed assessment & monitoring questionnaires that he recommended below:
As a general measure to assess and track changes in social anxiety severity, the freely available Social Phobia Inventory (SPIN) is the questionnaire recommended by the England & Wales NHS Improving Access to Psychological Therapies (IAPT) "outcomes toolkit". Interestingly David seems to prefer the Liebowitz Social Anxiety Scale (LSAS) probably because it makes a pretty full job of assessing both anxiety and avoidance.