Hack - noun. 1. Use of a resource for something other than its intended purpose. 2. An appropriate application of ingenuity.

Social hack - noun. 1. A hack whose underlying resource or intended purpose is social in nature.

The parent poster's hack is doubly social. First, the underlying medium is a social network. Second, the purpose is to reveal hidden information during socializing.

It's a hack because it's clever, unexpected, and small. It's social because it happens to be about a type of social interaction.