GMail contacts will no longer be automatically handed over to other websites and services, says Google.
The search firm will now only share user information if the site wanting access provides reciprocal data feeds to others.
The policy switch was primarily aimed at Facebook, said Google, complaining that the social network left users in a "data dead end".
Google said the policy switch would be implemented over the next few weeks.
Like many other web firms, Google lets others get at the data it holds on users of its many services via what is known as an Application Programming Interface (API).
Before the policy switch sites such as Facebook used Google's API to let their users automatically import GMail contacts so they could rapidly fill out their profile and find others that use the service.
Now Google will only give automatic access to GMail contacts to those sites and services that let others mine the data they hold.
In a statement shared with the Reuters newswire, Google singled out Facebook for criticism.
"We have decided to change our approach slightly to reflect the fact that users often aren't aware that once they have imported their contacts into sites like Facebook, they are effectively trapped," said Google.
Facebook has yet to comment on the row.
Gartner analyst Ray Valdes told Reuters that Google's decision is tied into its business ambitions.
"Google needs to evolve to become a big player in the social Web and it hasn't been able to do that," he said.
Analysts also suspect that Google's decision is related to the deal struck between Facebook and Microsoft that allows user data to power the Bing search engine.
Although Google has stopped the automatic siphoning of GMail data it is still possible for users to download their contacts and then can be shared with any web service.