Most people don't have that amount of RAM. What are the chances that a soft bit flip will cause silent data corruption that matters for a user with, say, 8 GB of RAM?
The idea of circles sounds fantastic on paper but it's simply too much work for the common user.
For most of the posts users would make, users would probably spend more time thinking about which circles to enable than actually writing the post. It's a headache and it leads to a poor experience, it feels like a hurdle, something you must do; it makes posting less natural.
Facebook on the other hand offers the same functionality but it's "buried" so you can use it at your convenience.