The UK’s all-in-one Internet radio platform Radioplayer attracted 5.7 million unique users in May and generated 22.5 million listening sessions. That comes down to about 3.95 session starts per unique user. These are the first stats to come from Radioplayer since its launch in late March.

The data shows that Radioplayer achieved a “15.7% reach of all Internet users” in the UK, according to Media UK managing director and “radio futurologist” James Cridland, who cites comScore figures from March 2011 (Cridland was involved in Radioplayer’s inception and is working the company on a future innovation workshop, but has “had nothing to do with the current implementation”).

The UK’s Rajar has found that Internet radio attracts around 5 million listeners in the UK a week.

“The UK Radioplayer is, in short, massive,” Cridland writes (here). But he expects the next set of stats from Radioplayer to be “a whole lot larger” as “a fairly large number of radio stations” joined Radioplayer after 5/16 (this set of data’s cut-off date).

Plus, Radioplayer announced today it will “soon open the door for smaller community stations and student stations to join,” reports the Guardian (here). A Radioplayer iPhone app is expected later this year.

Radioplayer launched on March 31 and now includes more than 200 UK radio stations including all of the BBC’s stations and many commercial stations (RAIN coverage here). The BBC has more coveragehere.