V. 1. Refers to the deepening romantic involvement between 2 individuals when both of them start to appear together in the thumbnails of their Facebook profile pictures. While anyone can share your profile pic, it takes a special person to survive the cropping and be part of the extremely limited 130 x 110 pixels that is a thumbnail.
2. Can also refer to non-romantic relationships, i.e. BFFs, bromance, etc.
Friend 1: Are Vangie and Phil, like, together? Their Facebook relationship status still says "single".
Friend 2: Yeah, but check out their homepages - they already share thumbnail space. It's official alright!