Facebook has long been a one-way valve. You put data in so you can connect with friends, and then you can’t get it out and use it the way you would like to, even as the company is trying to corner the market on your identity.

And Thursday night Google called out its rival on such data-portability hypocrisy: It banned the huge social network from allowing its users to connect their Gmail contacts to see who among them are also on Facebook to get the friending frenzy started.

Previously a Facebook user could give the social network permission to look through their Google contacts for e-mail addresses via Google’s Contact API. Facebook would match them up with user profiles to suggest people to befriend. And if those people weren’t on Facebook, you could send them an invite by e-mail to join—which is exactly how a social network goes viral.

But Facebook generally doesn’t reciprocate with other services—unless they are a partner. If you are also a Twitter or Buzz user and want to find out which of your Facebook friends were also using those services, Facebook will not let you.

Google—which has plenty of reasons to covet the rich mine of user data hidden behind Facebook’s walls—simply had enough.

“We have a data liberation engineering team dedicated to building import and export tools for users. We are not alone. Many other sites allow users to import and export their information, including contacts, quickly and easily. But sites that do not, such as Facebook, leave users in a data dead end,” Google said in a statement. “[W]e will no longer allow websites to automate the import of users’ Google Contacts (via our API) unless they allow similar export to other sites.”

Google isn’t protecting its Gmail customers, who would do this as a convenience to themselves, and who must give Google explicit permission to allow to happen anyway. It’s protecting its own long-term strategic interest in a relatively new battleground for what amounts to your online identity.

Facebook and Google are locked in a fight over who controls identity on the net, which Facebook is handily winning, with its Connect service that automatically logs you into some sites (and transfers your profile) and gives sites and now phone apps an easy way to let people login via their Facebook credentials. It’s a convenience that puts Facebook firmly at the center of the Web.

Google is likely picking pick this fight now because of an anticipated launch of a competing social networking service, rumored to be called Google Me. The fastest way to hit the ground running would be for Google’s 176 million monthly users to cross-index their friends among Facebook’s 500 million members. Facebook, the 800-pound gorilla, isn’t especially interested in giving a new competitor a leg up — especially one of Google’s size — and is making it clear it doesn’t want to let users have their data or play fairly when it comes to portability.

Facebook has a history of blocking its rivals and, until now, it hasn’t been serious challenged. In June, Twitter’s Facebook app gave users the ability to find which of their Facebook friends were also on Twitter, so that you could follow them or make a list of them to follow. Facebook quickly shut that down.

But there are a few big exceptions to Facebook’s policy: Yahoo Mail, the Flickr photo sharing site and Microsoft’s Hotmail, products from two companies Facebook has strong partnerships with (and Microsoft is an investor). So if you want to get your Facebook contacts into your Gmail account, you could create a new account on either of those services, import your friends e-mail addresses, then export them into a file and import them into Gmail. Doable for the dedicated, but hardly data portability.

But the exceptions clearly show there’s no real philosophical objection at Facebook to letting you export your friend’s e-mail addresses, and it’s pretty ludicrous to argue, as Facebook likes to, that you shouldn’t be able to download the e-mail addresses of people who friend you and choose to share that with you on their Facebook page.

Facebook tends to say, waving their hands in the air, that the data ownership issue is complicated. Facebook used that excuse to explain why its ballyhooed new data export feature lets you download all your pictures and your friends profiles into a webpage you can look at on your home computer, but does not include their contact info.

But that’s just spin.

Simply put, if someone is your friend, they don’t care that you have their e-mail address.

In response to a request for comment Friday morning, Facebook said that someone would reply soon, but didn’t do so by early afternoon.

The influential TechCrunch blog is portraying the move as a betrayal of Google’s openness principles. Frankly, that’s ridiculous. Any big service that plays fairly, like say Twitter, can still access the API. Startups won’t be affected either, according to a source close to the search giant.

“The rule will be enforced on a case-by-case basis,” the source said. “Google doesn’t want to disadvantage smaller sites that have limited engineering resources. The last things the company wants startups to do is have to write import and export APIs.”