For years, Facebook users have been able to search and invite Google contacts to join their social network through a single mouse-click. But ever wonder why the reverse was never true: why isn't there a button allowing Google users to import Facebook friends into their contacts list?

Back in September, Google CEO Eric Schmidt wondered about this during an appearance at a Google Zeitgeist conference. And in response, last night Google made a subtle change to the Terms of Service in its Google Contacts API, as first reported by TechCrunch.

The change means Web sites will not be able to automate the import of a user's Google contacts into their own database, unless they reciprocate the exchange.

"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," Google said in a statement.

This doesn't mean you can't import Gmail contacts onto Facebook, managed by Facebook Connect, you'll just have to add them automatically or export Gmail contacts to your hard drive first.

Yahoo and Hotmail have reciprocity deals with Facebook where contact information gets exchanged both ways, and are still available through Facebook Connect.

Facebook did not respond to a request for comment.