Fix migration issue from bug 809694: need to set provider as active when migrating previous state, r=markh

browser/modules/Social.jsm

@@ -123,6 +123,7 @@ this.Social = {
         if (active) {
           Services.prefs.clearUserPref("social.active");
           currentProvider = providers[0];
+          currentProvider.active = true;
         }
       } catch(ex) {}
     }