2.9%. And that's loosy-goosey because HBO has no advertisers to satisfy so they don't need to get Nielsen to count their viewers so nothing that's reported is quantified.
The last Superbowl pulled a 47.4. That's considered to be an average of 103 million viewers, despite the 304 million tv viewers that Neilsen believes in. But then, the demo for "super bowl" is "people who breathe" instead of "adults 18-54" which most of network TV goes for. Colbert makes people happy with a 0.42, which is barely a million people.
It's funny. People point to the amazing ratings Battlestar got and they peaked at 2 million viewers. I've had shows where 2m viewers was the death knell and we knew we'd be gone in a couple weeks. Firefly couldn't muster 5m viewers and it was the most expensive thing on Fox.
"Who Shot JR" was 38.2 with a 61 share at 91 million viewers... of 228 million people in the United States.