PhasmaFelis and mikeash have all matches mutual for the top 20, 30, 50 and 100. Are there other users like this? If so, how many? What's the significance of this, in terms of the shape of the graph?

tablespoon is close, but has a missing top 50 mutual (mikeash). In some ways, this is an artefact of the "20, 30, 50, 100" scale. Is there a way to describe the degree to which a user has this "I'm a relatively closer neighbour to them than they are to me" property? Can we make the metric space smaller (e.g. reduce the number of Euclidean dimensions) while preserving this property for the points that have it?