Mulligan estimated that 17 percent of Spotify’s active users are paying for the service, which would mean that the company has around 3.4 million paying subscribers. Growth has been accelerating ever since Spotify launched its advanced social features at Facebook’s f8 developer conference. Spotify grew 4 percent in May of 2011, according to Mulligan, but eight percent since the end of April alone. His verdict: “Facebook integration, coupled with launching in the US has turbo charged Spotify’s growth trajectory.”

Still, Spotify is no wunderkind, and growth alone doesn’t promise success. Mulligan compared Spotify’s growth with that of Pandora, the now-defunct Imeem and almost forgotten digital music pioneer Last.fm:

Still, Mulligan estimates that Spotify could gain up to eight million paying subscribers within the next 12 months. The company is looking to further kickstart that growth with its app platform as well as a new Pandora-like radio feature within the latest version of its client – something that’s apparently also in the works  at competitor Rdio.

A Spotify spokesperson called Mulligan’s numbers “speculation,” but added: “We are very happy with our growth in both daily users and paying subscribers.” Spotify’s most recent publicly released data still references 10 million active users and 3 million paying subscribers, but those numbers were first publicized before the company’s U.S. launch. in January.

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