Micro-blogging service Twitter is considering charging companies that use the service to market their products.

Co-founder of the service Biz Stone told Marketing magazine: "We are noticing more companies using Twitter and individuals following them. We can identify ways to make this experience even more valuable and charge for commercial accounts."

However, in a blog post, Stone then claimed Twitter had simply been "thinking out loud" and that Twitter would remain "free to use by everyone-individuals, companies, celebrities, etc".

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"What we're thinking about is adding value in places where we are already seeing traction, not imposing fees on existing services. We are still very early in the idea stage and we don't have anything to share just yet despite a recent surge in speculation. When we do, we'll be sure to let you know."

Twitter is proving very popular with businesses. Dell has 80 different Twitter feeds and about 11,000 followers. The company revealed last year that its 'Twitter sale alerts' have added around $1m in revenue.

Recent research by web analyst Hitwise revealed Twitter is now more popular than Digg's social news aggregation service, while traffic to the micro-blogging service has increased tenfold over the past 12 months. A number of celebrities use the service on a regular basis, including tech-fan Stephen Fry, who recently used the service to keep himself entertained while stuck in a lift in London's Centre Point for over 40 minutes.

See also: Twitter 'spitters' increase social networking spam