With the most recent episode, it's only been three days since the dome appeared. Sure there might be some people who would ration right away, but the majority of people would not have been thinking about it for the first few days. It looks like the ratings have stabilized. It got 10.8 million viewers for the most recent episode which I think is pretty even with the manhunt episode ratings. Under the Dome is easily the most watched show right now for total viewers.