How much can we genuinely keep private in the 21st century?

This question is inspired by an answer on my question in Social about advertisements.

It seems that our lives are tracked by giant internet companies, by the government, and by others in the 21st century.

Serious question: how much can we really keep private?

I’m not going to qualify the question further, because I really don’t know the answer.

Please note this is in the General Section. Please keep discussion on topic.