It is much easier for 'bottom feeders' to exist outside of walled gardens (e.g. Facebook, Google). That doesn't mean that most of the 3rd party ad industry are

It is much easier for 'bottom feeders' to exist outside of walled gardens (e.g. Facebook, Google). That doesn't mean that most of the 3rd party ad industry are 'bottom feeders'. The negative side of ads is much more pronounced amongst the 3rd party ad industry because there are easier attack vectors in the open market than in walled gardens. There are large 3rd party channels that have clean inventory, honest clients, and perform well for advertisers (Taboola, Outbrain, Criteo, AdRoll, etc).