This is much less the case now that anyone, be it a company or organization or even a solo organizer, can link a Facebook group to a public Facebook page. (It isn’t immediately obvious how to do that, though, which perpetuates the discovery problem.)
Didn't know you could do that. Til.
I found most of my meetups through Facebook.
A lot of people don't have Facebook or limit the usage.
Fair. #anecdata :)
Facebook groups pluses: lots of users.
Facebook groups minuses: not discoverable on the open web.