There are now 100 million people actively using Twitter, the company announced today.

If this news sounds vaguely familiar, it’s because the five-year-old microblogging service reached 100 million user accounts awhile ago. These new numbers refer to active users, which CEO Dick Costolo defines as people that sign into Twitter and use the site at least once a month.

In April, Twitter publicly confirmed that it had surpassed 200 million user accounts, a number that has presumably grown since then. This means that at least 50% of Twitter accounts are sitting dormant, according to the company’s own numbers.

There’s no telling how many of these accounts are completely abandoned, as opposed to simply being irregularly maintained. Still, these numbers suggest that while Twitter has done a good job of signing up new users, it could be doing a better job of keeping their attention.

The company unveiled some other interesting numbers during Costolo’s “state of the union” presentation this morning, which Search Engine Land liveblogged.

Some of the stats:

The service is being used to publish an average of 230 million tweets per day, which works out to more than a billion tweets every week.

Of Twitter’s 100 million active users, about half of them log in on a daily basis, a number that the company says has grown 105% in 2011.

Mobile continues to be a huge contributing factor to Twitter’s success, with 55% of users accessing the service from their phones.

Of course, not everybody who looks at Twitter is a registered user. Lots of people check out users’ profiles or come across tweets in search results. In total, Twitter.com sees more than 400 million monthly unique visitors in a month, which is about double the number of the site’s registered users, abandoned accounts and all.