Steven Englehardt and Arvind Narayanan of Princeton University spent a year investigating web tracking of the top million sites and over 90-million requests, and it discovered that the top trackers were Google, which had several domains, including gstatic, google analytics, and doubleclick in the top three, in addition to google.com. googleapis. Facebook and twitter feature in the top ten.
All of the top 5 third parties, as well as 12 of the top 20, are Google-owned domains. In fact, Google, Facebook, and Twitter are the only third-party entities present on more than 10% of sites. Web Census Shows Top Trackers are Google, Facebook, and Twitter [webtransparency.cs.princeton.edu]
When you look at the different industry sectors, news sites are top of the tracking tree.
It's really worth reading the report to gauge the extent and depth across sectors, and to get a better understanding of the breadth of user tracking. I know many here that are one side of the fence limit and to block as much tracking as possible, and I know others that utilise tracking.
As a webmaster it's important to understand your audience, but, to what extent is tracking going too far?
The tool used by the researchers is published as open source if you want to investigate canvas fingerprinting.