Also students are highly reliant on it. These types of loose connections but strong network effects are hard to break.
However... > Way cheaper than sending out a snail mail newsletter or even managing an email list.
This is true less frequently. Facebook used to be a good organic channel to build, but now FB is forcing more payments, by forcing ads to FB pages, and then again for boosting posts on those pages. Plus, the costs are rising.
> Groups of people who had lost contact in high school have gotten back in touch in retirement
I experienced this too, but I also had a controlled test, since I wanted to track down some high school friends who weren't on Facebook. Turned out not to be so hard, and caught up with a bunch of other old friends along the way. :)
Sometimes the alternatives offer other advantages we don't see if we frame the problem as replacing what we have now exactly.