  Although community detection has drawn tremendous amount of attention across
the sciences in the past decades, no formal consensus has been reached on the
very nature of what qualifies a community as such. In this article we take an
orthogonal approach by introducing a novel point of view to the problem of
overlapping communities. Instead of quantifying the quality of a set of
communities, we choose to focus on the intrinsic community-ness of one given
set of nodes. To do so, we propose a general metric on graphs, the cohesion,
based on counting triangles and inspired by well established sociological
considerations. The model has been validated through a large-scale online
experiment called Fellows in which users were able to compute their social
groups on Face- book and rate the quality of the obtained groups. By observing
those ratings in relation to the cohesion we assess that the cohesion is a
strong indicator of users subjective perception of the community-ness of a set
of people.
