Credit: <a href="http://sketchbookcity.blogspot.com/">Jason Wyatt Frederick</a>

A common reaction to last month’s 20th anniversary of Nevermind from critics and laypeople alike was nostalgia for a time when there was such a thing as a demographic-­crossing smash hit like “Smells Like Teen Spirit.” The cynical Adbusters type might see this as Stockholm syndrome-style longing for the comforting embrace of a marketing-­driven monoculture, but it also speaks to the way people appreciate music—for most of us at least, the experience feels incomplete if we’re not sharing it. Hence the existential horror with which the iPod and its little white earbuds were greeted in some corners—because the device was small, fashionable, and loaded with far more music than the Walkman and its descendants, people could and would wear it for huge chunks of the day, isolating each with his or her music.

The ubiquitous combination of iPod and iTunes (and other similar pairings of MP3 player and software) has made solo listening a bigger chunk of people’s overall listening time, and cuts them off from any music they might overhear. There are exceptions, of course—if you’re the type of person to share a set of earbuds with your bestie, for instance, or to play your tunes full blast off your cell phone on the bus—but more likely than not your listening is largely asynchronous with everyone else’s. The half dozen people wearing iPods on any given train car might share not a single song in their iTunes libraries. And quite a few of us perceive this isolation as an absence, at least judging by the number of products and services that have arisen over the past few years to attempt to introduce an explicitly social element to the consumption of digital music.

YouTube and Instagram have had huge success mating photo and video sharing to social-networking services, and musicians who want to open up direct lines of communication with their fans have a plethora of online outlets to choose from, with SoundCloud poised to take the top slot formerly held by MySpace. But so far there hasn’t really been a killer app that lets listeners connect with one another in more or less real time.

Last.fm, Rdio, and similarly socially minded streaming sites have never reached critical mass with the public, probably because their user interfaces aren’t especially welcoming—and their use of off-putting jargon (“scrobbling“) certainly doesn’t endear them to less tech-savvy newbies. The app SoundTracking looked like it had the potential to become the audio equivalent of Instagram when it was released earlier this year—point your phone at a speaker, and it’ll identify what you’re listening to and allow you to post the song’s artist, title, and a sample link to Twitter or Facebook (along with a picture, a comment, and even your location). But that takes a number of steps and about a minute—small barriers that also seem to have hobbled the social aspects of relatively popular song-identifying apps like Shazam and SoundHound. Apple’s attempt to get social by grafting Ping onto the iTunes ecosystem in September 2010 was a mess in a dozen different ways—not least because the developers removed Facebook integration shortly after launch, robbing the network of an easy way to build a user base.

Recently, though, there have been a couple of breakthroughs on the social-listening front. Turntable.fm opened its beta version to the public in May, and though at the time it looked like little more than a neat toy for music-minded geeks, it’s turned out to have serious growth potential. Users select a cartoony avatar, then take it to rooms where they can either chat and listen to songs selected by a panel of DJs (represented by a table full of laptop-wielding cartoons) or join in and start playing music themselves. As far as rights to the music are concerned, Turntable.fm seems to exist in at best a gray area, but the RIAA hasn’t tried to zap it yet.

Turntable.fm’s owners argue that playing tracks through the service falls under the definition of fair use as established in the Digital Millennium Copyright Act—and because that law only applies to the U.S., they shut down international usage. So far this claim seems to be satisfying rights holders, probably in part because Turntable.fm is paying royalties through SoundExchange (it’s a free service, but it’s attracted investors) and has publicly stated that it’s interested in signing further licensing agreements. The service claims to have built a user base of 600,000 without any advertising, and if it delivers on the promise of that early growth, there will be bigger payouts for rights holders down the road—still more incentive for them to refrain from quashing Turntable.fm now.

The genius of Turntable.fm is its simplicity. Anyone can DJ. Anyone can vote on whether the DJ’s selection is “lame” or “awesome.” (“Lame” votes can get a DJ booted.) Anyone can start a room, and there are no limitations on a room’s theme—it can be dedicated to “filthy dubstep” or simply provide a spot for coders working on a project together to hang out (there’s a chat feature built in) with a shared soundtrack. Entering a Turntable.fm room is sort of like tuning into a college radio show where you’re able to communicate in real time not only with the DJ but also with other listeners. Plus you can fire the DJ if you don’t like the music.

The Turntable.fm experience is what developers like to call “sticky”—using it is intuitive and potentially addictive. It’s already inspired real-life parties designed to emulate a Turntable.fm room, including a couple called Turntable.irl at Beauty Bar this summer. The service’s user base is still tiny compared to, say, the tens of millions Last.fm says it’s attracted, but the simple sociability of it could easily trigger a Twitter-size explosion in popularity among casual users.

In even bigger news for social listening, two weeks ago Facebook ended years of speculation by finally unveiling its plan to more thoroughly embed music into its user experience. Instead of making a head-on run at the iTunes Store, the plan simply provides integration with existing music platforms such as Rdio, Rhapsody, and MOG. Facebook seems willing to let competition among them define which music experience will become the de facto standard, and the smart money is on Spotify. In the months since Spotify debuted in the U.S. this summer (it’s already popular in Europe), it’s accumulated more than a million and a half users, some paid, some using hobbled free accounts.

Spotify is one of the few competitors to iTunes that looks able to replace it, at least for the nongeek. Its stand-alone application works almost exactly like iTunes, but augments the music on a user’s hard drive with a massive streamable catalog, and both are searchable through one text box. It’s dead easy to use, and the mobile versions work similarly, providing access to the same music.

The platform debuted here with Facebook and Twitter already baked in—right in the app’s main window you can post songs you play to Twitter, look up your Facebook friends’ favorite artists, and so on. The Facebook-Spotify overhaul two weeks ago deepens the link: now the only way to use Spotify is via Facebook (preexisting Spotify users are excepted), and if you agree to the new terms of service without adjusting the settings on your Spotify and Facebook accounts, you give them broad permission to exchange data.

The upside to this symbiosis is that Spotify listens can appear in your Facebook news feed—the same one that lets you know when your friends have uploaded a photo or linked to an article. This can add up to a flood of music, and with just one click you can hear to the whole album the track is from; another click opens up a back-and-forth with the original listener and other Facebookers. Spotify’s large and growing user base and the intuitiveness of its Facebook integration—you have to wonder why things haven’t always been this easy—make it a shoo-in to dominate Facebook’s music initiative.

Plenty of people are irked that Spotify will automatically post updates on what you’re listening to straight into the list of your activities on your Facebook profile page, but Spotify is already rolling out a new version of its app with a “private listening” mode that disables this feature. (It’s also relatively simple to disconnect your Spotify account from Facebook.) And good thing, too—social listening may be a net positive in aggregate, but not everybody wants his friends to know about his late-night Tegan and Sara binges, or for a potential employer to see that the last song he listened to was DJ Assault’s “Panties on the Ground.”