Twopcharts is a third party service that monitors the activity on Twitter servers.
They have started an unofficial countdown to 500 million accounts on Twitter network. And as per their estimates, Twitter should reach that milestone in around 40 days.
The service claims that Twitter is currently adding more than 10 accounts per second.
And at the time of writing this article, Twitter had around 465 million accounts.
This of course does not mean that Twitter had this many users. The service allow single users to open and operate multiple accounts and then there is the problem with spammy accounts.
Still it is a pretty good number to see the ever growing popularity of Twitter on the web.