Firstly, as others have pointed out, these companies collect information on people who have never opted in.

Secondly, they purposely set up a walled garden such that you can't see some things unless you make an account. Like say, the thousands of businesses with no website but only a Facebook presence. Or reading political discourse directly from politicians on Twitter. Or, as is very often noted, the easiest place to get support from companies seems to be on social media.

The latter isn't really easy to solve. One can say who cares, don't join then. But it can put you at certain disadvantages.