Speaking at the Web 2.0 Summit, in San Francisco, yesterday, Costolo said that of the micro-blogging service's 100 million active users, over 50% log in daily.
Costolo also revealed that Twitter is seeing 40% quarter-on-quarter mobile growth, and the release of Apple's iOS 5 has tripled mobile sign-ups since last week. Twitter is directly integrated into the OS.
When asked about the company's current valuation, Costolo reportedly said: "Let's just call it an even $8 billion."
Costolo revealed the active user numbers for Twitter for the first time at a press conference last month. At the time it was noted that the number of active users is half the number of registered users.Costolo also said at the time that as much as 40% of active users don't actually say anything on Twitter, and only log in to see what others are saying.
However, he maintains that Twitter is going from strength to strength, saying the service will outlast Facebook and Google+ because of its reliance on simplicity. According to Costolo, it is the site's simplicity and ease of use that contributes to its growth.
"We think we can reach every person on the planet, we think the way to do that is to simplify it," said Costolo, adding a subtle dig at the bevy of new features rolled out by Facebook and Google+.
Created five years ago, Twitter has changed very little – sticking to its 140-character formula. At the summit, Costolo said managing and filtering the volume of users and tweets is an “engineering challenge”.
"We have lofty ambitions, we want to be part of the fabric of every communication in the world," said Costolo.
According to Costolo, efforts to monetise the service have been very successful and 'Promoted Tweets' are working better than they had hoped.
Promoted Tweets was launched last year, followed by the recent introduction of promoted tweets in user timelines. Costolo said the company is looking into other ad formats, including multimedia, in the near future.
As with the roll-out of promoted tweets, Twitter will move cautiously in introducing new advertising formats. Costolo said different ad formats will be adopted, depending on how users react to any changes.
Twitter's approach contrasts somewhat with Facebook's recent major changes to its user interface. Despite the user backlash for some of the new changes, Facebook still dominates the social networking space with a reported 800 million active users.
The growth of Twitter and the entrance of Google's social platform Google+ has, however, cast some doubt over the future dominance of Facebook.
At the Web 2.0 Summit, former Facebook president Sean Parker said its “power users” have moved to Twitter or Google+.
At Google's recent results presentation, Larry Page claimed that Google+ currently has over 40 million users.
After a three-month closed field-trial, the service was opened to the public on 20 September. In that week, traffic to Google+ surged by 1 269%, according to online tracker Experian Hitwise.
Web analytics firm, Chitika Insights, has, however, published a report saying Google+'s growth spurt was short-lived and traffic on the site dropped by 60% to roughly the same level of activity it recorded before going public.
"It would appear that although high levels of publicity were able to draw new traffic to Google+, few of them saw reason to stay," said the report.
Chitika believes there are two driving reasons for the lack of interest in Google+. The first is that the supply of users for social media sites is limited, and the second is the lack of unique features to ensure competitive advantage.
Google engineer Steve Yegge also recently accidentally shared a private memo blasting Google+. While Yegge quickly removed the post, it had already been shared across Google+.
"Google+ is a knee-jerk reaction, a study in short-term thinking, predicated on the incorrect notion that Facebook is successful because they built a great product," wrote Yegge.
"But that's not why they are successful. Facebook is successful because they built an entire constellation of products by allowing other people to do the work. So Facebook is different for everyone."
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