"We don't have a shingle out on our front that says 'Twitter: For sale.' We're not for sale and we haven't been. We're very, very interested in building an independent company."

That echoed the message of Twitter's Chief Executive Dick Costolo who in interviews with the Los Angeles Times and other newspapers has pledged to keep Twitter independent.

Such speculation has swirled around Twitter for years. On Monday, Costolo denied the rumor that Twitter was in talks with Google after a keynote presentation at the Mobile World Congress in Barcelona, Spain. He did not answer a question about Facebook.

Stone also said: "We're not valued at $10 billion. That's just what people are writing in the newspapers."

Twitter was valued at $3.7 billion in a $200 million round of funding in December.