I'm not sure those can be separated as easily as you point out: SMEs who buy some ads on Facebook, but spend far more by making their page active, headhunters w

I'm not sure those can be separated as easily as you point out: SMEs who buy some ads on Facebook, but spend far more by making their page active, headhunters who have a paying account on LinkedIn, can be both considered as models where basic usage is free while more expert features are paying or where network externalities are decisive, and agents pay to benefit from their centrality.