What they should do is offer two options: either user-pays at some minimal amount per year (e.g. $5-$10) or advertising-funded where the user can choose to follow certain companies or brands.
The trouble is, the advertisers want exactly those users who would be willing to pay money; they're high-value eyeballs. By allowing their most valuable users to opt-out, they devalue themselves as an advertising venue. Of course, they also decrease the available impressions if a significant number of people opt out, making them a less attractive venue again.