"People on the street can do that too, I better walk with a paper bag over my head."
But there is no one person on the street that can do that for all other people on all the streets on the planet. Concentration of information in one "brain" is the problem, not that not everything is top secret. There is a difference in the possible consequences between some neighbour on the street seeing you and someone having a database of all public movements of all people.
Also, don't install or use the FB mobile app (good thing it sucks)
There are browser extensions that hide G+/FB/Twitter buttons, it's a good idea to use them
" Even if you never posted anything, they can easily work out your age, gender, sexual orientation and political views. "
Why is this being said with a surprised tone? Oh and this happens in real life too, you don't need fb for that.
"Guess your age and gender"? People on the street can do that too, I better walk with a paper bag over my head.