Twitter, the world’s most popular microblogging site, is already making money and must become even simpler and more unified across platforms in the future, CEO Dick Costolo told the Mobile World Congress in Barcelona yesterday.

Costolo didn’t fully answer questions about the company’s business model and its ability to monetize its fast-growing popularity.

“Every time I have an interview I am asked when we’re going to make money. The answer is we’re already making money,” he said.

But, he provided no real details on plans to monetize Twitter.

“Twitter quoted figures on healthy growth and use, which is good but not surprising. What it didn’t provide was concrete details on how effective its nascent businesses are proving to be in driving revenues,” Eden Zoller, analyst at telecoms consultancy Ovum, said.

As for the growth, it is impressive. During Super Bowl XLV last week, Twitter registered around 4,000 tweets per second towards the end of the game, compared to 27 during the same event in 2008.