The reality is while there's a vocal minority, and that it may be subjectively worse, the design was absolutely A/B tested and performed better in whatever KPI's Facebook chose as their goals (such as ad revenue, session length, engagement metrics). That's the simple answer.
Full disclosure: I haven't logged on Facebook much in the past couple years.
Very possible that the majority of people who truly care about these things "haven't logged on Facebook much in the past couple years". I very much fall into that category. There are so many awful things about it that the UI is the least of my reasons for drastically limiting usage.
I killed my Facebook accounts a while ago but walked past my partner's station while she was doing some Facebook stuff. I was a bit surprised how bad it looked... Tonka Toys meets White Mode AOL.
If people haven't logged in for the past years, they should never be the population considered to judge wether the new design is good or bad. OC's and your opinion are meaningless to the discussion if you are not a user.
I logged in for the first time in a few years a month or so ago to delete my account permanently, and was surprised how terrible it looks. It's got a real kindergartener vibe.
I certainly hope they didn't use this metric... I can't find anything anymore so of course it takes me longer! I still struggle to find things under Microsoft's "new" Office ribbon...
Instead of session length they could use time to success and number of successes. A success being dwelling on content for a certain length of time or interacting with it (commenting, liking). I guess these fall under GP's "engagement metrics".
Full disclosure: I haven't logged on Facebook much in the past couple years.