Twitter this week registered its 500 millionth account, as 10 new accounts are registered each second.

As of 11am this morning Twitter stat site Twopcharts said that Twitter has almost 5,000,700,000 users, and predicted that the micro-blogging service would hit 600 million in three and a half months.

Twitter was created in March 2006, but launched only in July 2006, so its growth is staggering by any standards. It had only 300 million active users as recently as last year. Going further back, Twitter had around 400,000 tweets posted each quarter in 2007, and 'only' 100 million tweets per quarter in 2008.  Yet by February 2010 that number had jumped to 50 million tweets... per day. As of just under a year ago, around 140 million tweets were posted each day.

Twitter itself hasn't commented on the 500 million users story, and there is some controversy about the statistic. For instance, we don't know how many of the users are active, or how often those that are active are used. Signing up for an account is one thing, populating it another. According to stats Twopcharts published a month ago relating to the first 400 million Twitter users, more than half (53 percent) of all Twitter accounts have never been posted to, and only 16 percent have been used in the past month.

No matter, Twitter clearly has a huge userbase. The question now is how valuable that is, and how Twitter can monetise its audience. Early attempts at advertising through promoted Tweets have proved underwhelming, and any attempt to sell all the user data Twitter has garnered are sure to prove hugely unpopular. See all internet news.

Follow PC Advisor Editor Matt Egan at @MattJEgan