In an interview with The Telegraph newspaper this morning, Twitter CEO Dick Costolo admitted that he's still trying to figure out Twitter's "long-term purpose."

Costolo, who took over for Evan Williams last month, said that he's "currently trying to define" what Twitter is going to become, and noted that users have come up with a lot of past innovations like the hashtag.

Sources say that despite its fast user growth and the introduction of new revenue sources like sponsored tweets and selling access to data, Twitter still isn't booking a lot of revenue. On stage at Web 2.0, Williams didn't offer any revenue numbers for what he called the "promoted suite," saying only that demand was outstripping supply.

But that's a little bit misleading, as the Promoted Products Platform isn't open to general advertisers through a self-service platform (like Google's AdWords, for instance). Instead, Twitter's still selling it only to a select group of test advertisers, and everybody else is supposed to fill out a form for more information. It's easy for demand to outstrip supply when supply is artificially constrained.

Costolo is supposed to help navigate this transition, but it's a hard one: if Twitter sells too many ads, the signal-to-noise ratio will drop and users will abandon the service. He told The Telegraph that he'll have more information to share about Twitter's long-term plans in the "near future."

In addition to defining how Twitter's supposed to make money, Costolo is also dealing with growing pains. He noted that the company now has 325 people and is trying to figure out how to grow without creating crippling bureaucracy, and that it has a "scalability problem" that will soon require more data centers, particularly in Japan.

Apologies: this article originally miscredited the source. It's The Telegraph.