As suggested by Tanya Snook I ran the Tweet Reports, but in searching I found Tweet Reach which had a reporting tool that determined #a11yyow tweets reached 15,004 people. This was via searching for info within 50 tweets on December 06. 22 Tweets, 22 RT's and 6 @replies. The combined twitter exposure for the event was 37,057 impressions. I don't know if that's good or not, however I've attached the reports from both.