Much is being made out of projections by Twitter tracking service Twopcharts’s projections that Twitter will activate its 500 millionth account next month, but a closer look at the microblogging service’s growth shows anything but a steady rise.

Twitter passed the 200 million mark last February and then, on May 18, it reached 300 million accounts. That’s 50 percent growth in roughly 90 days and the milestone gave Twitter stock as one of the Big Two (at the time) social networks.

But something happened on the way to 400 million accounts. It would be 162 days before Twitter reached the 400 million account mark on Oct. 28. Depending on what day in February Twitter reaches 500 million users, it will have taken the company 95 to 124 days to add 100 million users: better than the climb from 300 million to 400 million, but still not as fast as growth 200 million to 300 million.

Milestones like this tend to be arbitrary, but they’re especially so in Twitter’s case. The company only counts accounts that have been opened and does not factor in accounts that have been closed by Twitter or the user. By Twopchart’s estimates, about 12% of all Twitter accounts ever opened have been closed, meaning that by the time the 500 million number is reached, there may only be about 440 million existing Twitter accounts.

Facebook is expected to surpass a billion users in August, which is all the more staggering because of the company’s strict policy requiring people to register under their real names. Twitter, on the other hand, lets users open multiple accounts, and even spoof accounts or accounts for services.

There are other layers of complexity in figuring out Twitter’s reach: by its own estimates, the service has 100 million “active” users, but it’s safe to say the majority of accounts are inactive or rarely used. Twopchart has an interesting blog post full of data, including an analysis of what has happened to the 400 millionth account.

@AdenMo has tweeted just once since becoming the account number 400 million, has just three followers and is only following one account. That’s a fairly typical account, based on Twostory’s analysis and their estimate that only about 20% to 25% of new Twitter accounts convert into active Twitter accounts.

“The vast majority of new accounts only has a limited amount of followers and following, and based on the amount of tweets sent, and the submission date of last tweets, probably only a maximum of 20%-25% convert to active new twitter users, with only about 10% accounts that actively send tweets,” Twopchart said in the blog post.