Twitter CEO: We have 100 million active users

Twitter now has 100 million active users, which is pretty impressive when you realize that it was a little more than half of that just earlier this year.

The growth of the micro-blogging site is nothing short of astonishing. CEO Dick Costolo gave a press conference at the company’s headquarters where he explained how phenomenal the site has become.

He did not describe exactly what constitutes an active monthly user, but said that more than 50 million of them log into their account every day. Many Twitter accounts are automated or not operated by an actual person.

Costolo said he expects to add another 26 million active users by the start of 2012.

So far, at its peak, Twitter reported users sending nearly 9,000 tweets per second, following the announcement of Beyonce’s pregnancy.

Of course, there are still throngs of people who don’t have a Twitter account but still check into the site on a regular basis to see what everyone else is saying. Costolo said there were 400 million unique visitors to Twitter.

There have been numerous advancements to the still-very-simple messaging site, including making it easier to share photos and locations. It has also become an incredibly important service for both the public and private sectors. Twitter is now a marketing/customer service extension for many companies, and also acts as an official channel of dialogue for politicians and government agencies.

It seems like even with such massive expansion Twitter still has lots of room to grow.