One person can click to see different subforums on those sites, and often does. One FB/Twitter, you can only see stuff posted by your friends, friends of frien

One person can click to see different subforums on those sites, and often does. One FB/Twitter, you can only see stuff posted by your friends, friends of friends, specific people whose names/usernames you know and have searched for, groups you have chosen to join, and ads they show you. The other 99.99% of content is invisible to you.