A year ago Boardwalk Empire premiered to 4.809 million viewers and a 2.0 adults 18-49 rating (its season highs). Its December finale (which wasn’t its lows) scored 3.294 million viewers and a 1.3 adults 18-49 rating. Lows of a 1.1 adults 18-49 rating happened a couple of times, including the season's penultimate episode. It saw a viewer low of 2.572 million on 10/10/10.
To spare the complaints of "HBO doesn't care about demos, it cares about subscribers!" (which is true) this poll is for total viewers. I also get that typically half or more of the viewers usually aren't coming from the week's premiere telecast at 9pm, but we don't have that data to do any comparisons.