Starting today users can listen to music from the four major labels and construct and share playlists with their MySpace buddies, with advertisers picking up the tab for all the free tunes.

However, if users want to download songs or transfer the music to their iPods, they’ll have to purchase the tracks from Amazon MP3.

With more than 120 million users mostly consisting of teens and young adults, MySpace Music represents what might be one of the largest ready-made online audiences of aggressive music consumers ever to be targeted by the music industry. However, getting that same audience to switch from illicitly swapping unauthorized tracks on P2P networks may be the biggest task of all.

But even if MySpace users forgo buying the tunes, the advertising paying for the free spins is sure to pick up some of the slack with major ad clients like McDonald’s, Toyota and State Farm Insurance footing the bill.

The majors also hold small equity stakes in the new music service, with shares reflecting each label’s market share. This means Universal Music Group has the biggest stake in MySpace Music while EMI holds the smallest chunk.

Although MySpace Music was expected to launch this month, there were some last minute deals to be made before the service debuted. EMI was the fourth and last major-label to commit, signing on just hours before the launch date was announced, reports Reuters.

Aside from major label content, MySpace Music also signed a licensing deal with The Orchard, one of the largest distributors of indie music. Music publisher Sony / ATV, a joint venture held by Sony Corp and Michael Jackson, is also onboard.

“When all is said and done, we will have the richest catalog of content on the Internet,” said MySpace COO Amit Kapur.