It might be more country specific (I don't live in the United States, nor in a metropolis), and specific to the art/music scene and venues I'm interested in, but Facebook really covers most bases for me. And I often discover great events because some Facebook friend is interested in them. Some dev meetups are crossposted to meetup.com or eventbrite.com, but only some, and Facebook aggregates both for me. There's even a group for local IT events.

I understand that things happened before 21st century, obviously, there's still posters, word of mouth, newsletters (in some cases, but they're not that popular over here), but these days, at least where I live, event organizers are using Facebook as a core platform to spread awareness. I certainly didn't know about a lot of these events before I joined Facebook.

And there's events like bike rides, hiking trips etc. organized by individuals... on Facebook.