Let’s get some obvious stuff out of the way:

1. Listening to music is fun.

2. Listening to music with friends is more fun.

3. Listening to music with friends, free, is the most fun.

The past week has contained a lot of discussion about Spotify, the new-to-the-United States free-music service that can stream almost any song to you, instantly.

Spotify is great. I’ve been using it for the past few days and love how I can try out new artists with no risk as well as call up old favorites that I never bothered to rip from my CDs or purchase online.

But Spotify is basically a more expansive iTunes, a better (and cheaper) Rhapsody. I get greater variety and lower costs, but the fundamental thing I’m doing — selecting music and playing it — hasn’t really changed.

There’s another service out there, also new, that has upended how I listen to music. In addition, it has changed how I think about social networks and what they are capable of. The service is called Turntable.fm, a free site that’s been up and running for about a month or so and is starting to get some attention.

Social Networks The way people connect digitally.

Turntable.fm lets you play and listen to music with friends. That doesn’t sound like much, but once you’ve experienced real-time collaborative D.J.-ing with your pals, you will very likely find it as revelatory and delightful as I have.

Turntable.fm sets you up with an avatar, which can drop in on any of the hundreds of listening rooms users have set up on the site. You can also set up a listening room of your own. You can either passively listen to what other people are playing or, if there’s an open spot on the D.J. stage (up to five people can D.J. in one room), you can join in and add your own music to the mix. Tracks can be selected from Turntable’s vast catalog of music, or uploaded from your PC. You can like and dislike songs, post comments to the room and follow other D.J.’s whose sounds you like. If you have friends on Turntable.fm (it links into your Facebook account), you’ll see where they are when you log in.

So that’s how Turntable.fm works, but how you use it is another matter altogether.

The way most people first use the site is by going in and out of rooms on their own. It’s a great way to discover new music. You go into a room with a genre that appeals to you and listen to what people are playing. It’s like, well, radio — but maybe a little more quirky and eccentric (definitely more quirky and eccentric than commercial radio).

If popping in and out of listening rooms were all there were to Turntable.fm, it would still be a great site. It solves the musical cul-de-sac problem that many people have: they listen only to the stuff they’ve downloaded. New music is hard to discover. Turntable’s listening rooms let you break out of that cycle and expose you to new acts.

But what takes Turntable from “great” to “amazing” is what happens when you start a listening room with friends, because it’s then that things start to get giddily satisfying.

It doesn’t matter if you start a room with some nearby cubicle mates or with friends scattered across the globe: what you begin to realize — almost instantly — is that taking turns playing music with friends is a kind of communication. One song leads to another. Music, enjoyable in and of itself, becomes a sort of shorthand when played among people who have shared memories attached to it. Someone plays a song that was popular when you were college, then another friend plays another song from that same period and — just like that — you’ve traveled back in time. It’s like you’re all sharing in the same inside joke.

And this feeling is reflected not just in the choice of songs, but in the comments that friends post. Someone digs a lost hit out of the crates and the message board lights up with comments from friends (O.K., my friends) saying things like “!!!!!!!!!!!!!!!!!!!!!!!” and “my head is exploding.”

Using Turntable can even help you make friends with people you only know in passing. If you’re D.J.-ing in a room with co-workers, you may never have realized that you and the guy across the floor both like Afropop. Now you have something new to talk about at the soda machine.

Turntable.fm reveals another advantage music has: You can listen to it while you’re doing something else. Other forms of online interaction (chat sessions, video chat, gaming) require your full attention. And that’s great — if you have that attention to give. But if you’re D.J.-ing on Turntable.fm, you can stock your list with as many songs as you want and the site will play the next one when your turn comes up. Then you can go back to working, hanging out or doing whatever it may be while you are listening not just to your music, but your friends’ music as well.

Other music sites and services have tried to merge songs with social networks. Spotify allows you to share songs and playlists with others — you can even allow friends to modify shared playlists. But you are still listening on your own. Apple has social features on iTunes with Ping, but that just lets you share information with other Ping users in a static, asynchronous way.

Turntable.fm gives you a live, as-it’s-happening exchange that’s akin to having all your friends over for a party. When your old college roommate surprises you with that Deee-Lite song the two of you used to rock out to, you know it’s more than just music: it’s a message.