GoT premiered to 2.2 mil that grew to 4.2
Boardwalk Empire premiered to 7.1 mil
I find the whole business side very intriguing and I'm going to go off on a tangent here.
They, HBO, do have a very interesting dynamic/business model when you think about it. Having to entice viewers to pay $13 a month and movies are much less of a draw these days.
They are always going to have crime/cop and comedy shows. They're relatively easy to make, have a wide audience appeal, and they already have fans of this genre's $13. So, if they want more market share they have to expand into other entertainment factions.
It's interesting to me that they went into high fantasy to try and pull in more viewers. (They tested the waters of fantasy with True Blood) I thought for sure they go sci-fi or even crime/sci-fi e.g. an X-Files type of show before going high fantasy.
But as a fantasy fan I think they made an excellent choice. By and large the fantasy market is extremely untapped in movies/shows, and usually when it is tapped into it turns into a huge joke. However, when it's done right they go on to become unbeatable franchises; Lord of the Rings, Harry Potter, Star Wars, the only thing that can come close to these in the non-high fantasy is probably James Bond.
Anyways, just some of my musings. It will be interesting to see how it all plays out and if this encourages HBO to take other leaps into different factions for an even wider overall audience. I'd looove an HBO x-files type show.