Rank- League- Average Attendance Per Game of League
1) NASCAR- 200,000 - 300,000
2) NFL- 67,738
3) FBS- 46,971
4) MLB- 32,717
5) MLS- 16,464
6) NBA- 17,394
7) NHL- 17, 265
8) AFL- 12,415
9) WNBA- 8,039
10) NCAAB- 5,277
However, fan attendance doesn't tell the entire story. 1) A lot of fan will watch games on TV, so it's entirely possible that the sport may have a high attendance but low popularity elsewhere- or vice versa 2) Some sports (i.e. the new Cowboys Stadium) may just have bigger arenas than other sports
Luckily, a Harris Poll helps to tell the entire story. They polled people to see what there favorite sports to help determine what the most popular sports in America are (click to enlarge):

Therefore, the most popular sports in America looks a bit different than attendance numbers:
1) NFL
2) MLB
3) FBS
4) NASCAR
5) NHL
6) NBA
6) NCAAB
6) PGA
9) MLS
9) PBA
9) Horse Racing
So now back to the original argument that sparked this post: DME was wrong about the immense popularity of hockey in America. Hockey is MUCH more popular than either golf or tennis and absolutely should be considered one of the top 4 in America.
Sources:
-Harris Poll
-Sports Attendance
-NAASCAR info