Peter Kafka interviewing Daniel Ek of Spotify at D: Dive Into MobileDaniel Ek, CEO and cofounder of online music-streaming service Spotify, has 750,000 paying subscribers, up from 320,000 in March. It now has more subscribers than any other similar service in the world, he said.

What he doesn’t have: A firm date on when the European startup will launch in the U.S.

Spotify had previously said it planned to launch by the third quarter of 2010. Then it said it would launch by the end of this year. In an interview at the D: Dive Into Mobile conference, AllThingsD media reporter Peter Kafka pointed out that it’s December 7, and pressed Ek on whether the company would stick to the 2010 plan. (That’s Kafka, left, interviewing Ek, right, in the photo above.)

“I won’t commit to a specific date,” said Ek. “It just takes time” to negotiate contracts, he said at a later point in the interview.

His cause likely has not been helped by the involvement of early Napster employee Sean Parker, now an investor in Spotify through the Founders Fund venture-capital firm. Parker recently said that Spotify would “finish what [Napster] started” in pushing the music industry to provide its wares online.

Spotify’s subscribers in Europe, where it succeeded in obtaining rights from record labels to stream music at economically favorable rates, pay 10 euros (US$13) a month for unlimited access to Spotify’s library. But users can also stream a limited number of hours of music a month for free.

“Most people in the world do not pay for music today,” said Ek, pointing to countries like China. “The music industry should be worth a lot more” than the $15 billion it is today, implying that if it embraced services like Spotify, it would increase its value.

“There’s no reason why music can’t be more popular than photos online,” said Ek.

If that’s his argument, it’s taking longer than expected to win the labels over to his point of view.