The below was emailed to PM contributors. As far as I see you would have to opt out of Extended licenses on PM to exclude your images from the Social Media license.

"We get a lot of inquries for using images within social networks. Consequently, we are introducing our new "social media licence" as part of our extended licences. The additional licence fee will be 10 Euro per image.

With our new extended licence, customers can now post images within social networks like facebook, twitter, pinterest and google+:

"Social Media Licence: The content may be used and published within social networks (so called �share� functions, e.g. within facebook)."

If you do not agree to offer your images with an optional extended licence, you can deactivate your extended licences in the contributor portal -> "policy management". We will introduce the new social media licence not earlier than 14 days from today." (Email sent on May 31, 2013)