Portable Social Networks
Today, most social networking sites are 'walled gardens' and silo's, so that individuals must maintain a great number of different social networks, unable to share that information.
So the concept of 'portable' social networks refers to the portability of this type of contact and relationship data.
Example
The Friend of a Friend project at http://www.foaf-project.org/:
"The Friend of a Friend (FOAF) project is creating a Web of machine-readable pages describing people, the links between them and the things they create and do.
FOAF is about your place in the Web, and the Web's place in our world. FOAF is a simple technology that makes it easier to share and use information about people and their activities (eg. photos, calendars, weblogs), to transfer information between Web sites, and to automatically extend, merge and re-use it online."
More Information
Some commentary and proposals on the issue: