Google is keeping its promise of making all of its data, all of your data actually, open and available so you can simply take it and leave Google if you want. It's the direct opposite of what most (Facebook) other big companies do, but it's a great feature for users.
Google users have had Google Takeout for this; the tool already supports several types of data and several Google products.
Now it's adding Google+ Circles data, but there's a twist. You can export all of your Circles data, but only to another Google account. That may seem like a limitation – it is one, obviously – but it's not really. The fact is that there's nothing else you could do with the Circles data other than using it for another Google+ profile.
"A while back we announced plans for a tool that would help you transfer your circles from one Google+ account to another and ensure that your followers are automatically directed to your new, preferred profile. Today, anyone can visit Google Takeout and click 'Transfer your Google+ connections to another account'," Google' Ronald Ho wrote
It could be argued that Circles data could be useful to other services. After all, the relations data is much more important than a simple list of contacts for any social service, especially a social network.
Google could have made it available in a standardized way which could be read by other apps and imported into other sites, though it would have been a hassle and would have depended on other sites adding support for the data.
Still, you can already export all Contacts data you have on Google, be it from Gmail, Google Talk, your Android phone or Google+. There's also some data linked to Google+ that's already available, +1 button data for example. Profile data can also be exported along with your stream.