Why do you care? EVERYTHING is monitored by the government including all your email, contacts, etc.
If you are not doing anything wrong, there should be no concerns.
Advertisers monitor our every move on the internet, purchases, etc. and then use that information to place ads which may interest us everywhere we go.
I'm sure Amazon and all other businesses monitor us to some degree to sell us what we may need.
For example, on 10/24/2012 of the most popular 1,000 sites Doubleclick.net tracked 681 sites, Scorecard research.com tracked 475, Adnxs.com tracked 439, Quantserve.com tracked 409, and Admt.com tracked 391 per this link:
You have to be registered with the NY Times to read that link.