Privacy settings

The default settings on any service were chosen by that service, for reasons that suit the service.

That is not necessarily sinister, but it is worth remembering that nobody picked them with you in mind. The privacy settings page is where you find out what was decided on your behalf: who can see what, what gets used to build a profile of you, what is shared onwards. Most people never open it, which is what makes the default so valuable to whoever set it. Ten minutes in that screen, once, on the handful of services you actually use, is unusually good value.