According to the FT between 10% and 30% of all online marketing spend is on fraudulent views. Clicks to sites are generated by 'bots programmed to behave like humans, often via malware infected machines. 

It seems the dream of perfect targeting is further away than ever - particularly on third party sites. 

To make matters worse, real humans often use ad-blocker software so these metrics probably under-state the problem.

All the more reason, one could argue, to build your own community rather than hope to use someone else's.