jaxtrlogo121.pngJaxtr, a Silicon Valley company that offers Internet calling, is crowing about its growth rate. Five million people have used the service since it launched, up from half a million users in July, it says.

To be clear, this is not the same as its monthly active users, which the company didn’t disclose. So many users who experimented with the service once may be counted in its number, but may no longer be using the service.

Indeed, Jaxtr’s fiercest competitors, Jajah and Jangl, report growth too, but are vague with specifics.

Menlo Park, Calif.-based Jaxtr says its growth is evidence that it has a superior interface that helps drive adoption.

With Jaxtr, you sign up and get a URL link, then you send that link to a friend. They click on the link and enter their phone number. Then, their phone rings, your number rings, and the call connects. Jaxtr provides a local telephone number, so you can make calls overseas while paying domestic rates. You can also link all of your accounts to one number.

However, its competitors’ interfaces are — at least to us — also relatively easy to use.

To be frank, VoIP calling services still aren’t as intuitive as normal landline or mobile phone services, at least for most people. Mass distribution has not been easy for any of these startups.

The place where VoIP really stands out is when it is built into a more comprehensive suite of office communication services so you can easily record messages and store them with your email. Or, it can be used as a cheaper way to make international calls if you have friends and family overseas.

Jaxtr, Jajah and Jangl all offer widgets on social networks and other sites, in an effort to get the average Myspace or Hi5 user calling their friends on it. Jangl has cut business deals to make money: It provides anonymous calling services to dating sites like Match.com and the FriendFinder.com empire.

UPDATE: The company also says 45 percent of its users logged in to its site in the past 30 days — mostly to check voicemail. It also says most people make calls without going to the site, either through a widget or through their phone.

Jajah and Jangl may have sensed a shakeout among VoIP startups. They recently announced a strategic partnership, where Jajah will continue work in its calling infrastructure and Jangl will focus on building social web applications.

Still, Jaxtr claims to be growing so fast that it is facing scaling issues, necessitating more engineers. It recently poached Taneli Otala from MySQL, to be company’s new vice president of engineering.

Meanwhile, the other companies keep coming out with more features. Jangl now has an SMS service, for example.

Here’s a short video on how Jaxtr works.

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