By Jonathan Davies

Twitter's co-founder Jack Dorsey is "not satisfied" with its user growth, despite beating expectations on revenue and profits.

The social media giant posted revenues of $502 million in the second quarter, up 61% from the same period last year.

Twitter had 316 million active users during the quarter, but it was only up slightly from the 308 million recorded in the first quarter.

"We've got unbelievable brand awareness, but people are not clear why they should use it themselves," Jack Dorsey said.

He said the majority of new users come from its SMS fast followers - people who receive text alerts but don't need an account. Without these users, growth was just 0.7%.

Chief financial officer Anthony Noto said Twitter would launch a marketing campaign by the end of the year, aimed at informing people why they should use the social network.

But he warned that Twitter would not see "sustained meaningful growth" until it reached the mass market, which would take a "considerable period of time".