Google has finally launched its online music store, though it wasn’t able to get all the music labels on board in time.

Universal (and its new subsidiary EMI) were already known to be part of the project, while Sony was a surprise inclusion at the launch after earlier reports it hadn’t reached a deal. Warner Music — which makes up a little over 10 percent of the market by sales volume — is now the only major holdout.

Warner does have some major artists (both contemporary and “classic”) in its arsenal, so it looks as if Google Music is right on the border between unavailable music being a minor irritation and a factor that deters users from bothering with the service at all. By way of context, Google is advertising 13 million tracks, while iTunes has around 18 million.

Google Music is initially US only, with expansion likely slowed by licensing issues. It’s going for a price range of 69c to $1.29 per track, with no copy protection. Though its available on computers, the goal seems to be to make it “the iTunes of Android”: the service has been built directly into the Android Market so that music can be bought in the same way as apps (in other words, in a simple, impulse-friendly manner.) Customers on T-Mobile will be able to instead choose to have purchases added to their phone bill.

There are a few exclusives, mainly previously unreleased live albums, but the main selling point is, perhaps inevitably, social sharing. Whenever you buy a song, each of your contacts on Google+ will be able to play it once without charge.

There’s also a special deal for independent artists. They pay $25 to have a page on an “Artist Hub” and can then sell music at any price, with Google taking a 30 percent cut.

Early reaction to the service seems to be along the lines that there’s nothing wrong with it, but it lacks any true killer feature to help it stand out against established players, notably Apple.