Rdio has officially launched its promised free music streaming option that will give users in the U.S. on-demand, ad-free access to their 12-million song catalog. There's a limit on the amount of music you can listen to each month, and a meter at the top of the site will keep track of how much of it you have left in your account. Unlike Spotify, which gives a hard cap of 10 hours a month for their free account, Rdio isn't disclosing the amount of free streaming a person will get.

Rdio explains the free music limit like this: "The amount of free a person will get is dynamic and will be based on their own personal use and displayed in a meter in the corner of their profile. A heavy user will get more, but probably not as much as they will want and a light user will get more than they'd ever want."

The company says the free music streaming option will serve as a marketing tool for Rdio and a way to get people to try the product, which is why it decided to go the ad-free route instead of trying to monetize free accounts. Users who regularly hit the mystery cap can upgrade to the $5 per month option that lifts this limitation, or the $10 per month subscription plan that includes access to Rdio's mobile apps and lets you download music for offline listening.

There's also an Unlimited Family plan that offers 2 unlimited subscriptions for $18 per month or 3 unlimited subscriptions for $23 per month. Rdio users will also be able to share music through social networks such as Twitter and Facebook – though unlike Spotify, which requires a Facebook account, Rdio presents it as an option.

The free service requires just an email address or Facebook account to sign up. There's no credit card required unless you want to spend the extra cash upgrading to one of their paid subscription plans.