The two biggest names in webland, Facebook and Google, have had somewhat of a frosty relationship as of late.

The announcement of the potentially Facebook-rivalling Google Me wouldn't have been well received by Zuckerberg and the gang, and Facebook has also been keen to stick the boot into Google at times, by announcing a partnership with Microsoft and Bing.

But the latest revelation reveals just how untenable the web giants' relationship is becoming. Google has announced that it is blocking Facebook from accessing its contacts API - a key aspect of Facebook's social reach.

When people sign up to Facebook they were given the opportunity to add friends via their Gmail contacts (as well as Yahoo and Windows Live contacts). From now on, new users will not get this option.

And, although Facebook is around 600 million strong now, the Gmail network is also humongous, so the block is a major setback in terms of growth for the social network.

Google has blocked Facebook as a result of a revamp of its terms and conditions which now state that organisation's wanting to tap into its API must reciprocate (basically, they must offer a similar service). Facebook doesn't let anyone touch its contacts database, so it's found itself out of the Google loop.

Google released a statement to TechCruch that read:

"Google is committed to making it easy for users to get their data into and out of Google products. That is why 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.

"So 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 users will still be free to export their contacts from our products to their computers in an open, machine-readable format - and once they have done that they can then import those contacts into any service they choose. However, we will no longer allow websites to automate the import of users’ Google Contacts (via our API) unless they allow similar export to other sites".

When Facebook began it relied on email addresses to spread its network (originally Harvard addresses and then eventually expanded to other universities and into the mainstream). And although the reliance on email addresses has decreased rapidly, a block from Gmail will still be greeted with some worried looks at the Palo Alto HQ.

It's Facebook's move next, it will be fascinating to see how it responds.