06/01/2011 04:15 pm ET Updated Aug 01, 2011

Twitter CEO Dick Costolo: 'We're Growing Like A Weed'

Twitter is doing pretty well for itself, according to CEO Dick Costolo.

Costolo sat down at the All Things Digital conference to talk about Twitter's growth, how it regards social on the web, and plans for expanding search.

According to Costolo, the microblogging service handles a billion tweets every six days. (It took 3 years to send the first billion.) And in the past year, mobile usage has spiked 150 percent, with 50 percent of users accessing Twitter through third-party clients rather than the site itself.

"We're growing like a weed," Costolo said proudly.

And that weed is doing very well for itself. While Costolo declined to detail Twitter's profits, he did say that Twitter is a "remarkably successful business." Eighty percent of advertisers renew with Twitter, and though the site had 150 advertisers by the end of last year, Costolo says they're now up to 600.

At yesterday's D9 sit-down, Google Chairman Eric Schmidt listed a "Gang of Four" influential Internet platforms and didn't mention Twitter. Costolo, however, didn't seem too miffed about being snubbed--after all, Schmidt said that Twitter had the potential to break into the big four.

While Costolo wouldn't say anything about Schmidt's admittance that Google had failed at social, he did have some things to say about what social means to Twitter.

Twitter, he said, thinks about an "interest graph," rather than the social graph. He considers this to be a "very, very different way to think about identity." For example, you might follow Costolo on Twitter, but he might not follow you; similarly, Costolo follows the San Francisco Giants, who don't follow him.

Costolo also explained Twitter's improvements to the search experience within Twitter and explained that the site will introduce relevance-sorted results. In the future, Twitter plans to provide even better context in Twitter results.

But when it comes to searching the Twitter archive, Costolo says that a choice has to be made. "We have the full archive of Tweets," said Costolo. "But we have to choose between real-time search and super-fast archive search. We've decided to focus on real-time."

Read on to see a demo of Twitter's search improvements, as well as the site's new photo-sharing feature.