According to a recent analysis of over 80,000 health-related web pages:

• 91% of health-related web pages send HTTP requests to third party advertisers and data brokers
• 70% of these requests include information about specific symptoms or diseases
• Google collects user information from 78% of pages, comScore collects from 38% of pages, and Facebook collects from 31% of pages

Source: "Your Privacy Online: Health Information at Serious Risk of Abuse," Annenberg School for Communication, University of Pennsylvania Press Release, February 20, 2015, https://www.asc.upenn.edu/news-events/press-releases/your-privacy-online-health-information-serious-risk-abuse