I've played in both USHL and Junior A in Canada and I can tell you for a fact that hockey especially junior and minor hockey in the US has grown astronomically.
I've actually looked at the U.S. Census numbers on participation in sports. This is based on a questionnaire sent to 10,000 households in the U.S.
Hockey (ice)
2004: 2.42 million, kids aged 7-17: 836,000
2007: 2.07 million, kids: 673,000
2008: 1.91 million, kids: 598,000
That's like a 20% decline from 2004 to 2008 in all age groups and a nearly 30% decline with children and youths.
Comparisons:
Baseball
2004: 15.85 million, kids: 8.23 million
2007: 13.95 million, kids: 6.88 million
2008: 15.17 million, kids: 7.27 million
American Football (tackle)
2004: 8.20 million, kids: 4.72 million
2007: 9.20 million, kids: 5.35 million
2008: 10.48 million, kids: 5.51 million
Basketball
2004: 27.85 million, kids: 13.04 million
2007: 24.15 million, kids: 11.87 million
2008: 29.70 million, kids: 13.29 million
Since these results are based on statistical sampling, there's obviously a margin of error here and that can explain some of the fluctuations, though none of the other sports suffered losses between 2004 and 2008 as they gained strongly from 07 to 08. It kind of makes sense given the demographic development, too.
Important of course to note, these aren't numbers of governing bodies or anything, so this is not necessarily an overview of organized play, any play is counted and based on a "Have you in the last year participated in sport X" type question. You might well have an increase in active junior hockey participation and a decrease in overall hockey participation.
An interesting to take from those numbers is that adult rec activity seems to be a bigger factor with hockey than with the other sports. Hockey is also a "rich people's sport" with 1.2% of Americans with a household income of over 75,000 dollars playing hockey (12% with basketball, 3.8% with football, 6.4% with baseball) as opposed to 0.4% of Americans with incomes under $15,000 (10.5% with basketball, 4.3% with football, 4.8% with baseball). As you can see the numbers in the other sports barely or not at all favor the wealthy whereas in hockey it is a very pronounced difference. That probably isn't shocking given the hockey demographic and the high equipment costs but it's still nice to see it in stats.
I find those numbers pretty plausible overall, the demographic shift from the North to the South and the increase in non-white population would correlate with the decline of hockey as a "people's sport" and becoming a more organized affair of the middle-class and above.
For the NHL both numbers would be of importance, the organized juniors might produce more pros than they used to, but the overall participation number is maybe more indicative of the overall fan base of the sport and league.