There are several levels of privacy:
1. Companies using the data for market research
2. Companies advertising goods and services
3. Individuals researching you and finding "unauthorized/private" data
4. The government researching you / keeping dossiers.
All I have to say is:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated ...
4th Amendment to the US Constitution
This YouTube video is an excellent introduction to Google+.