Television is a very different experience in the age of video on demand. Even if we're watching the same kind of content – say, 30- to 60-minute scripted dramas and comedies – we watch it differently. Even if it's broadcast live (usually through a cable or satellite box) to our television sets or streamed through a game console, media player, set-top box or smart TV interface to that exact same television set, that subtle difference changes everything.

With live television, we flip; with video on demand, we binge. This means that shows have to catch and hold our attention in very different ways – not just over the commercial, but from episode to episode, season to season, and from television to videogames, Facebook, or whatever else might capture our attention on a web-connected device.

Crucially, these differences mean that we gravitate to different content. Many of the most popular, highest-rated shows hold relatively little appeal if seen through a video-on-demand streaming service. The converse is also true; some of the most successful streaming shows (like NBC's Community) struggle to find a corresponding audience on broadcast television.

How much of this boils down to sheer availability? Longtime blogger Tristan Louis recently posted a list cross-referencing the top 50 scripted broadcast series for 2010-2011 (in terms of total viewers, as determined by Deadline.com) with their availability for streaming or download through Netflix, Hulu, Amazon and iTunes.

In short, the more popular the show, the less likely it is to be available on streaming services. (At the limit is The Mentalist, last year's third most popular show, which along with a handful of other shows, isn't even available for purchase through Amazon or iTunes.) This pattern is especially true for Netflix, whose streaming catalog is heavily focused on movies and television's backlist. But even Hulu Plus is less likely to have full seasons of the most popular TV shows available – partly because so many of them are on CBS, the only major network without a substantial partnership with Hulu. (Hulu links to CBS shows available at CBS.com: Louis assigns this an availability rating of "partial," which I think is a little too generous.)

As long as CBS is getting huge viewership and great ad revenues for its tentpole franchises like CSI, NCIS, The Big Bang Theory, it doesn't have a real incentive to make them widely available for digital streaming for anything less than top dollar. Forget diluting the audience: It's about getting back comparable value.

But there's another interpretation possible here, based in part on a handful of anomalies in this trend. A few very popular shows have relatively broad availability on streaming services: Modern Family on Hulu Plus, Glee, which is available on both Hulu Plus and Netflix, plus The Office, Grey's Anatomy, Bones and How I Met Your Mother (ditto), along with Fox/Hulu Plus standouts Family Guy, House and more.

These shows have a few things in common: their audiences are younger; most are constructed with long story arcs that build over entire seasons; and they have an unusually large number of highly devoted fans.

Modern Family, for example, may "only" have the 13th highest viewership of scripted TV in total audience, but it's number one for viewers 18 to 49: Only live sports and reality television beat Modern Family in that demographic. The rest of the demo's top 10 is filled out by Grey's Anatomy, Glee, The Office, Family Guy and House, with only a few CBS shows (The Big Bang Theory, NCIS and Two and a Half Men) mixed in.

Younger viewers are gravitating toward shows that are available digitally, and both Netflix and Hulu are working to deliver those series to them.

Andy Forssell, Hulu's Senior Vice President of content, has a handy formula for his approach to acquisition, for both existing network shows and original content (emphasis mine):

We'll look for content that's beloved not beliked. The content that really pays off and punches above its weight in our ecosystem is a show that somebody's going to see and then they want to go e-mail five of their friends or get on Facebook and post about it... In our world, we'd much rather have Community than Two and a Half Men, and I don't mean that as a criticism of Two and a Half Men. It's great for advertisers that want to use that show as a proxy to get to this big audience. But for us, we're much more excited about Community because while it's a smaller audience, it's an audience that self-organizes online. They'll not only tell their friends to go watch it, they'll spend time convincing someone on a bus to watch it.

This devoted, digitally fluent fan base is also much easier to target with dedicated and personalized ads. They'll also pay $8 each month for Hulu Plus, and convince their friends to do the same.

In so many ways, the strategy for services like Hulu Plus and Netflix today resembles that of cable channels like HBO a decade ago: Find niche, must-have content, whether it's Glee or Community or The Sopranos and Sex and the City, that keeps individual viewers paying up every month.

In digital video, the only thing that might be better than a big, amorphous crowd tuning in is a highly engaged community that wouldn't think about changing the channel.