"Missing link" media refers to the second tier of established, so-called "mainstream" media, only one rung below the corporatized "legacy" media. They are generally accepted by the mainstream as credible sources but they have considerably less financial power than the top rung legacy media.
The missing link media will be the first to suffer from competition from by citizen journalism and alternative media sources on social media websites. They will either be absorbed into their legacy media counterparts or will fold under the strain.