Grindr is a free social networking application for Apple’s (AAPL) iPhone, iPad and iPod touch that uses the devices’ geolocation technology to help gay and bisexual men see photos, chat with and hook up with partners that happen to be nearby.

What Grindr’s more than 1 million users may not know is that its owner, Nearby Buddy Finder, LLC, has stored their user names, passwords, locations and unique device IDs and is sharing some of that data — including phone identifier, city, zipcode and latitude/longitude coordinates — to three different mobile advertising networks.

One of those networks, Millennium Media, lists 11 types of information about people that developers may transmit to “help Millennial provide more relevant ads,” including age, gender, income, ethnicity, sexual orientation and political views.

Grindr is one of the 101 apps highlighted in Your Apps Are Watching You, a Wall Street Journal investigative report published Saturday that details how iPhone and Android apps are breaching the privacy of smartphone users. And Grindr isn’t even the leakiest. According to the Journal, Pandora and TextPlus 4, share their user data with eight partners each. Four apps, including Bejeweled 2, actually give out phone numbers.

The report, by Scott Thurm and Yukari Iwatani Kane, includes an interactive tool that displays the types of  data being collected and with whom it is being shared for each of the applications the Journal tested — including the paper’s own iPhone app.

As Andrew Lewis once quipped (now available as a T-shirt), “If you are not paying for it, you’re not the customer, you’re the product.”

Below: A WSJ.com video in which Julia Angwin explains to Simon Constable how this works.

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[Follow Philip Elmer-DeWitt on Twitter @philiped]