It seems that the adults in their 20s have settled down on Facebook and no longer have an interest in building up their personal Facebook Empire.
5 years ago I was always posting statuses, adding pics and sending friend requests. Now, I can't remember the last time I met someone and thought, "Hey, I should send them a friend request when I get home later." Many young adults, including myself, have downsized their Facebook Empire, deleting friends, old photo albums and old statuses, and henceforth keeping a more limited presence.
There are 2 reasons why many have downsized their FB presence:
(1) They realized that less is more. Posting less makes you seem like you have more of a life.
(2) They've experienced repercussions from their activity. Maybe they didn't get that job because the nosy HR lady found their Facebook profile and saw that they "like" a pro-life page. Or maybe they drunkenly messaged a girl from their high school and she defriended them.
Less of an FB presence = less metadata = less $$$ for FB,
which is why FB makes it so difficult to downsize your presence. Ever wanted to delete all your old statuses? You have to delete each one individually. It could take you an entire day. And then you'll realize that FB temporarily hides a lot of your old statuses. You think you deleted them all, but when you back onto Facebook you'll see old statuses that you didn't see before.