Dave Berri notes that despite the sense you frequently get of the NBA declining in popularity, attendance is actually way up:
In 2006-07 the average NBA team attracted 726,954 fans during the regular season. And this was the all-time record. Let me repeat. Last season the NBA - which Shanoff says is declining in popularity - set an all-time attendance record. And this is a per-team average record (of course they also set a record for total attendance).
To put this mark in perspective, 20 years ago - during the peak of the Boston-LA rivalry — the NBA’s per team average was only 550,190 fans. Across the past 20 years, while the U.S. population has grown 23.8%, NBA attendance has grown 38.7%.
So more teams and higher average attendance -- that seems pretty good. What's probably going on is that thanks to the proliferation of media everything has more of a niche vibe than it used to. Back before most people had cable, anytime anything was on television at all it was somewhere between 1/3 and 1/7th (depending on where you lived) of the total things available to watch on TV and there were no DVDs or websites or OnDemand offerings to provide further options. So anything that got above a certain "it's on television" threshold automatically acquired a certain air of mass relevance that, these days, is very hard for anything to achieve.