Let's say in a pretend world where Facebook gives a shit about anything (so, they combat marketplace scams, they don't randomly ban users, they faithfully show you posts from friends, they don't extort businesses) they want to protect privacy. So they close the site off, because scraping means there's no privacy. If I scrape your profile and export it to any number of sites, then it's not private. Ergo: closed. Sign up to view things, so we know who you are
But of course in the actual world that's not what they do. They purely want to do this to get more people to sign up so they 'have more users'. You can tell, because of who MZ is. You can tell because of how aggressive their own AI scrapers are on other sites. I've even heard they made a tool for Myspace that would enable you to transfer your contacts over to Facebook - but not the other way around