Given the way Google has been using high-pressure onboarding through several of their existing customer bases, I'm not surprised. And "visitors" likely counts people who got linked to a single post.
I'm surprised there hasn't been a legal challenge to this.
What Google is doing is very much reminiscent of what got Microsoft into so much trouble. Using a monopoly in one area to extend into another.
Not when you have your entire team/company using gmail as a paid service. And when you factor in the part about the user being the product in social networking, basically you're forcing a paying customer of one product to become a revenue stream in another.
Monopolies find themselves in trouble over this all time, just like when a customer service agent from your credit card company or local energy conglomerate passes you onto a sales rep from a third party without your knowledge. It's how they scam the old, ignorant and uninformed.
tl;dr: you cannot change your bookmarks when you have no other options.