I disabled my Facebook account 4 years or so ago and didn't miss it. That said, I now have a daughter with an extremely rare genetic disorder. Although there are a number of databases out there specifically to help parents find other families with rare genetic disorders, we didn't find anyone until we resorted to Facebook. This is only one anecdote, but from my personal experience thus far, Facebook is still unfortunately the best place to find a needle in a haystack.
This is my same experience. Son has JIA. There is a JIA-specific website that connects you to JIA-specific support groups.... but there are none active near me, and I live in a very big city. I expressed this frustration to my wife who uses Facebook often, and she joined an active JIA community in moments.
I guess now we have two anecdotes. That makes us a statistic, I think?
(edit: typo) FB has completely swallowed the Groups space. We are new parents and my wife connects with other new parents in a local group specific to our son's age cohort. FB Marketplace is also very useful, as other people have mentioned. I got off FB services a few months ago, but would join again if they had a standalone Groups app.
