Companies like Google can be a threat to privacy due to the information they gather and what they end up doing with it. Even though they’re providing a free service, there are more privacy sacrifices as your analytics data is tracked, stored and owned by them. This means they have access to data like internet activity logs (websites, pages visited, searches), which reveal a lot of personal info about our life, and work.
When using Google Analytics, Google knows all the IP addresses (and other browser unique identifiers which can be considered personal data) of visitors to your site. Through this they can then track the six other websites that person visited earlier that day, and the 50 websites she/he looked at in the last month. By re-using such visits log data tracked on your website they can enrich existing profiles for given IP addresses.
Google Analytics dominates the industry with a market share of 84%. Coupled with their other products that use tracking beacons, such as Adsense, you can see their immense ability to build an accurate picture of most visitors to websites.