Twitter is suddenly rife with complaints about ads showing up in the free version of Spotify, which recently saw the addition of new limitations about how people can use it for free.

On top of those limitations (which are, to be fair, still more permissive by a long shot than those enforced by any other free on-demand music service), users are now putting up with an apparent increase in audio/display ads on the service — in some cases including multiple ads between songs.

“Ads are targeted and delivered differently according to variables such as time of day and type of user and market, and we’re continually testing different ad products and groupings,” Spotify spokesman Jim Butcher told Evolver.fm on Friday. “It’s crucial we maintain the great user experience we’re known for while driving value for advertisers.”

A spokeswoman for a Spotify competitor alerted us to these complaints, which is admittedly a bit cheeky. We couldn’t confirm her claim directly, because it’s not possible to encounter ads with the U.S. version of Spotify that has been available to the press for about two years now, but a quick Twitter search verified that she wasn’t making them up:

Cwintzy: I see Spotify have started playing two ads between songs, like we wouldn’t notice or something!

PaulJIsaac: @PrinceBert The ads on Spotify don’t bother me. If I had an income I’d purchase all my music properly and leave Spotify for dust!

Corriengel: Okay, these Spotifyads are becoming ridiculous. Gtfo.

mrhaste: The guys who record the Spotifyads sound like Muppets. Not in the colloquial sense, they actually have Muppet voices

Bauza23 I’ve moaned before but 9 ads in a row? Spotify are really pushing for the whole subscription thing eh?

26mjw Spotify ads make me genuinely rageful.

_metricmeg: ”oh you can’t expect to have it for free” nobody has ever had it for free, that’s what the ads are there for. #spotify

MBlair22: @michaelwhite1 The base of Spotify was free listening & ads. Now itsads&restrictions. Surely they couldn’t have pissed of the audience more

[And finally, summing up what lots of people see as the reason for the ad increase...]

durietz: Is it just me or is @Spotify now playing ads 3x as often? The record companies must’ve really got to them.

Indeed, speculation has been circling for years that labels weren’t happy with the small number of ads on Spotify because they weren’t enough to convince people to upgrade.

“Unfortunately, nobody is upgrading to the premium models, which is the hope down the road,” said Amanda Marks, executive vice president and general manager of Universal Music Distribution, all the way back in ’09. “There’s almost no advertising in the free section [of Spotify], so there’s no impetus to upgrade.”

However, the ads have always been part of Spotify’s plan, and are likely there primarily to support royalty the free version (which now allows users only 10 hours of playback and 5x play of any individual song after the first six months). Still, as with all “freemium” services (including the popular interactive radio service Pandora) the removal of audio ads is one factor that leads people to upgrade to the premium version as over a million European Spotify already have, giving it more paying subscribers than any U.S. on-demand music service.

Spotify is likely to continue tweaking its ad-serving algorithm following this feedback, but nobody should expect ads to disappear completely from an “ad-supported” service.

So, why all this action? Spotify has been trying to launch in the States for quite a while now. Some speculated that it would happen in March, at the SXSW festival — the same place Spotify CEO Daniel Ek (wisely) wouldn’t commit to a launch date during our keynote interview last year.

It’s never a good idea to bet that Spotify is about to launch here. But between these new usage limitations and the evident increase in ads, there’s more evidence pointing in that direction than there has been in years.