Two things have launched in the past month or so that sent people into a frenzy to acquire invites. First, Google+‘s invite system had the internet community in a virtual freakout with people finding ways to game the system as well as bargaining for invites on eBay. Google eventually lightened up on the restrictions and now Google+ enjoys 25 million users and that number is steadily rising.

The second service to launch with invites was overseas streaming music service Spotify. The long-awaited U.S. release was met with excitement from the internet community. Basically, the desktop client allows users to stream millions of tracks for free, on demand. You can check a more in-depth run-through of the service here.

And now, less than a month after debuting in the States, Spotify looks to be expanding its subscriber base at a pretty quick clip.

According to All Things D and their “source familiar with the company’s operations,” Spotify has 1.4 million U.S. users enrolled in the free trial. Even more importantly, they already have 175,000 subscribers who have opted for the paid plans.

The paid plans allow for offline music, unlimited play, and mobile capabilities. The most expensive paid plan is $10.

This makes a conversion rate from free trail to paid subscribers of 12.5%, which is just a little below the 15% rate that Spotify said they had in Europe last year. Spotify has at least 1.6 million paid users in Europe. It’s unsure whether U.S. paid subscriptions will ever reach such a large number. Not only does the U.S. version of Spotify sport a larger music library with its free service, but the competition is strong in the States

Just to name a few you have Last.fm, Grooveshark, satellite radio like Sirius and Rhapsody.

But to put it in perspective, Rhapsody has 800,000 paid subscribers. They have been around for almost a decade.