Twitter co-founder Biz Stone has spoken about how the micro-blogging service plans to make money, suggesting that tools and services could create revenue streams, rather than advertising.

The company is developing "various add-on tools and services for the businesses and professional users of Twitter", reports Reuters, with some tools due to be added by the end of the year.

Such tools could include "lightweight analytics" and what's described as a "directory of commercial accounts" that would verify that businesses on Twitter are legitimate. Revenue sharing with mobile phone operators is another route the company could go down.

"There are a few reasons why we're not pursuing advertising - one is it's just not quite as interesting to us", Stone told Reuters.

"There are no people at Twitter who know anything about advertising or work in advertising. So we don't have anyone there to make or take those calls".

A message we've heard before, Stone also said that whatever the plans, Twitter would remain free for consumers and businesses.