More than 100 million users are active and log onto Twitter daily, or about half its registered members, and a growing number are doing so from a plethora of mobile devices, Chief Executive Dick Costolo told reporters on Thursday.

Costolo said the microblogging site was chalking up about 40 percent growth every quarter in mobile usage. now sees about 400 million unique visitors every month, a 60 percent leap from 200 million at the start of the year.

Twitter, which allows people to send 140-character text messages, or Tweets, to groups of so-called followers, is one of the Web's most popular social networking services, along with Facebook and Zynga and Google Inc's embryonic Google+.

Analysts have sought details however on how many of Twitter's users are active, a key metric in determining its business potential.

(Reporting by Alexei Oreskovic; Editing by Tim Dobbyn)