Social networking site Twitter has signed up its 500 millionth user, according to a monitoring site.

Estimates from Twopcharts stated the milestone was passed earlier today (February 23rd), with registrations already up to nearly 501 million, with over 13 new accounts being registered every second.

However, not all of these users are likely to make an impact on the number of messages being sent each day, as the site calculates around 20 to 25 per cent of users become active in the community, with just one in ten actually posting tweets, the Inquirer said.

"Only Twitter can tell what the exact number is and how often people log into the system," Twopcharts started, though the site itself has refused to comment on the figures.

PC Advisor added that according to statistics posted last month by the monitoring firm, 53 per cent of Twitter accounts have never been posted to.

Despite this, its popularity is likely to continue growing, with Twopcharts estimating it will sign up its 600 millionth user in around 103 days.

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