Meanwhile, if you're not logged in but you know the URL of a specific page, or conduct a Google search and get hits on "public" messages (open content), you can consume that content. For example, do a search for "octopus site:facebook.com" in Incognito mode in chrome (no cookies/not logged in) and you'll get results.
So, it's not TOTALLY closed. Just the front door is closed.
I was just considering making my homepage the Recent Activity feed, since it's an all-encompassing list of things going on. But then I realized, at least by default, that feed isn't visible to unregistered users... and it probably shouldn't be. Which got me to thinking... what if I made my xenforo-based site behave in a similar way to FB? Registration is free, and in my case you can get further perks with a Supporter upgrade... but from the onset, when you come to the front door, you have to register to get started. Use "remember me" to maintain a personalized experience.
Forum content, articles, etc. would be open for everyone to enjoy, with some exceptions. There are certain things (like member lists, and Activity Feeds) that are only available to registered members.
I am guessing robots.txt could be optimized to ensure search engines are able to find all the publicly-available content... but coming in through the root domain would not be very satisfying UNLESS you are a registered user.