Digital music streaming service Spotify started as an invite-only platform, but taking a page from Google+ this week, the Swedish-founded company has now elected to open its doors to anyone in the U.S. who wants to use Spotify's on-demand music utility.

By signing up for Spotify, U.S. users can enjoy free access to Spotify's library of streaming music for six months. The service will occasionally show ads during this time, but following the six-month free trial period, users still get 10 hours of free listening a month.

The company tried free registration before. Back in February 2009, Spotify offered free accounts to the public in the UK, but the surge of responses prompted Spotify to shutter open registrations and return to an invite-only policy.

Spotify will try free registrations again after announcing its new partnership with Facebook at f8.

We knew that the service had to be free to draw people away from piracy, wrote Ek in a company blog. To do that, we also knew that the service would have to be inherently social. There couldn't be a better place to do this than Facebook.

Under the new partnership, friends listening to songs through Spotify will appear in Facebook's new Ticker, which is an abridged mini-feed relegated to the upper right corner of your screen. From the Ticker, users can see what their friends are listening to, and even join their friends live and listen to the same song simultaneously.

On the main News Feed, users will also receive highly relevant stories about the music people like. If a number of friends have listened to the same song together, those songs will also appear in the News Feed in an attempt to attract more listeners and, for Spotify, registered users.

Furthermore, Facebook will also display a music dashboard, which contains everything anyone has listened to within your network, including a long feed of trending albums and songs.

Founded in 2006 by Swedish serial entrepreneur Daniel Ek, Spotify provides access to roughly 15 million songs. Users can search for artists, albums, titles, labels, and genres, and Spotify returns tracks from major and independent labels alike. The four major U.S. record labels have contracts with Spotify, but several other bands and labels do not for various reasons. For example, The Beatles catalogue is not on Spotify because the band has an exclusive digital distribution agreement with iTunes.

If you don't have Spotify already, you can sign up on Spotify.com, or through Facebook by clicking somebody's News Feed update. For $5 a month, users can listen to Spotify's catalogue on PCs sans advertisements; for $10 a month, you can enjoy ad-free music on PCs, and on iPhone and Android apps.

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