Voice-based microblogging has existed for a while with services such as Audioboo and iPadio, but new service Bubbly has started to get coverage after hitting 500,000 users within the first four weeks of launching in India.

The voice-based mobile phone service allows you to share an audio snippet (usually around 30 seconds) with your followers. And it follows on from Bubble Talk which has 100 million users using the service to send voice messages to other individuals.

Users follow by typing in a person’s number or access code and are then updated when there’s new content.It’s free to upload, and the company earns money by taking a cut of the airtime used to listen. And with Bollywood stars among the early adopters of the service, it’s not surprising many of the half million new users are teens and tweens. There’s quite a high level of engagement with each message getting listened to by about 50% of the senders’ followers.

But I don’t think this is the new Twitter.

Mainly because the number of content creators on Twitter is pretty low compared to content consumers, and that seems to be even more of the case with an audio format. Rising all the way to video, which I overlooked when I tipped the original version of Seesmic as the ‘next Twitter’ at an event a couple of years ago.

100 million might be sending messages to other individuals, but how many of the current 500,000 are happy broadcasting and hearing their own voice being played on the mobiles of countless other people?

So far Bubbly has launched in 3 of India’s 23 states, and plans to expand to India, Japan, Europe, the Middle East and Brazil.

In the UK it’ll compete head-on with two existing services, iPadio and Audioboo. Both also offer the opportunity to upload audio from your mobile or a landline and have that audio distributed to followers. Both are gaining users by offering an interesting service, and neither has scaled to Twitter-like heights as yet, despite some very compelling content. And the issue for all three is that people like the thought of being able to sample fame by writing some content or uploading some audio, but there’s a bigger fear barrier to overcome when speaking.

That isn’t a criticism of any of the audio microblogging services. I’m a fan of all of them, I know some of the people behind them fairly well, and I’ve finally installed the PixelPipe workaround to use Audioboo from a Nokia. But to call them the next Twitter is akin to saying live video streaming sites like Ustream or Justin.tv are the next Youtube. Different tools, different distribution and different effects.