Facebook suffered from 2 waves of "Eternal September"[0]:
wave 1: the addition of everybody outside your immediate friends and close family. You get the goofy uncle, the distant high school acquaintances, and all the work acquaintances from every previous job that submitted a "friend request" that you didn't reject out of politeness. The newsfeed gets polluted by nonsense such as forwarded memes, politics, and other junk.
wave 2: the corporate advertisers filling up the newsfeed that crowds out the desirable posts from your real friends & family.
The 2004 to 2008 period was probably the "magical" time for Facebook from a user standpoint. The initial wave of Harvard students had a beautiful user experience with TheFacebook. As for the new members joining Facebook today in 2018?!? Not so much.
Of course, ads are needed to pay for the datacenters so any social network like Facebook inevitably decays into a worthless waste of time.
All true, but the Eternal September for me came when I lost control of my feed to arbitrary ML algorithms attempting to maximize shareholder value whilst alienating the fewest users.
I think someone could recreate early FB on the blockchain and take a huge chunk of interesting people off of the two current awful choices. Because I do remember the time when it was fun to reconnect with people and follow their lives there.