It's not exactly a success (except from the most selfish and financial perspectives), it's more like proof you can abuse your users and get away with it.
I used facebook messenger to keep in touch with my family across the world. My family is incapable of using signal, whatsapp, sms, email, telegram, phone calls, or anything else.
I also use facebook to buy stuff on the classifieds. It's like craigslist except there's at least a picture of someone and a paper trail if they decide to murder me.
My neighborhood organizes events using a facebook group and we've been able to have festivals and organize groups to go fight against companies that want to industrialize the area.
Facebook is trash at what it was initially built to be, but it is still very useful.
What about Myspace? $800m in revenue from 115m users. The classic 'good product' story, right?
Facebook makes its money by leaving the feed an unorganized mess of random posts, leaving the user disoriented so that more eyeball time can be harvested for advertisements while the user searches for the content they want (or gets stuck mindlessly scrolling the infinite reel of videos and other content for that dopamine hit).
It's not lost on the army of six figure salaried engineers that a lot could be done easily to make the experience better. But that isn't in the interest of the shareholders. The quality is a feature, not a bug.
As for whether its beneficial to society is another question.