Music lovers (and especially music pirates) rejoice - Swedish music streaming service Spotify has invaded US and it has one mission - to become one lean, green, music machine with the best social features around and currently its free music service is on invite-only basis.

Spotify was launched in 2008 in Sweden by Daniel Ek and Martin Lorentzon, who wanted to battle music piracy and offer a platform which can be used by music lovers to listen to high, quality streaming music.

In Europe, Spotify has over 10 million users and more than 1.6 million paying subscribers, and is now the second single largest source of digital music revenue for labels in Europe (IFPI, April 2011), making sure that artists get a fair deal.

Spotify users can listen to a great collection of music by accessing its 15 million songlist and unlike rivals Pandora or Rhapsody, with Spotify, the users don't have to worry about skipping tracks they don't like because you choose the music in the first place.

With Spotify, you can also go social - share your favorite music with your friends and family members who use Spotify, simply by dragging and dropping your tracks or by sharing with them via Facebook, Twitter, email and SMS.

We believe that music is the most social thing there is and that's why we've built the best social features into Spotify for easy sharing and the ultimate in music discovery. Even if you aren't a total music freak, chances are you have a friend who is and whose taste you admire. I'm looking forward to connecting with some of you in Spotify and discovering some cool new tracks, said Spotify founder and CEO Daniel Ek.

And, the best thing about Spotify is that it can double up as a mighty music player. Once you download and install Spotify on your smartphone or iPod, you can wirelessly sync your MP3 playlists and listen to your tracks offline. Craving for more? Simply sign up as a Spotify premium subscriber and get access to Spotify's catalogue of over 15 million tracks. It's that simple.

Spotify is currently in its beta phase in the US and the lucky few can avail of any one of the three services Spotify offers:

> Spotify Free: With an invite, enjoy on-demand, buffer-free access to over 15 million songs on your computer, great social features, manage your own music files through Spotify, and sync with your cellphone or iPod. Features occasional advertising. If you're interested in the free account, click here to sign up.

> Spotify Unlimited: All the special features of Spotify's free service but with uninterrupted, ad-free access to Spotify on your computer. All for only $4.99 a month.

> Spotify Premium: The over-the-top all the music, all the time Spotify experience. Spotify Premium gives you access to all the music, all the time. Listen online or offline, on your computer, your cellphone and a whole heap of other devices. Enjoy enhanced sound quality and access to exclusive content, competitions and special offers. Premium costs just $9.99 a month (that's the equivalent of a few fancy coffees).

Now for the BD (billion dollar for the uninitiated) question: How does Spotify stack up against Pandora, Rhapsody and a bunch of other compteting music service?

No doubt Spotify's free account is good (though on invite only basis at the moment) and unlike Pandora or Rhapsody, with Spotify you won't encounter any buffer or will need to skip unwanted tracks, but there's one big deal-breaker - Spotify's free account allows just 10 hours of music per month and only five repeats of a single track. And that's like telling a thirsty man lost in the desert to drink with a spoon!

In conclusion, Spotify is here to stay but it'll face a tough challenge from Pandora, Rhapsody, Amazon Cloud Player, Rdio, Google Music, Slacker, iTunes (Match), MOG and the ilk. Especially Pandora, which recently unveiled a redesigned site, will pose a major threat with around 100 million users. Rhapsody, which offers a $9.99 all-you-can-eat music service, boasts of 800,000+ paying subscribers and is also a big threat.

No wonder, the company's quick to announce Coca-Cola and Sprite, Chevrolet, Motorola, Reebok, Sonos and The Daily as its exclusive launch partners.

However, a tie-up with Facebook or Twitter or even Google+ would have hit the sweet spot. A tie-up with Facebook seems most likely with Facebook founder and CEO Mark Zuckerberg already testifying, Spotify is so good.

Meanwhile, check out the cool Spotify promo video below: