Safari - web sites being added to my cache
Greetings! I have an IMac. I just upgraded to El Capitan and am using Safari 9.0. I do not use Facebook, Twitter, or anything Google-related, by design. Yet, whenever I am about to end a session on Safari and go into my Preferences -> Privacy and look at the Details of my website data, I always find a bunch of cookies and sites added to my cache that I did not visit and do not want. Many of them are related to Google, Facebook, Twitter, Yahoo, and various ad trackers. My questions are:
1) Are these things embedded in Safari 9?
2) If not, why do they keep invading my computer?
3) How can I prevent them from invading my computer?
I know it would be "easier" to just let things be, but I guess I'm just not ready to completely give up control of my on line experience to the likes of Google, etc.
Any suggestions would be appreciated.
iMac, OS X Yosemite (10.10.5)