What's wrong with organising a meeting by email or WoM?
I help organize events for a small social group. Most of our members don't plan that far ahead, and even many of the regulars don't use email. We're also always looking for new faces, or for occasional members to join us at an event.
Meetup gives us a calendar, email list, reminders, attendance tracking, event tracking (history and also comments and ratings), and visibility to people who might be interested but never heard of us.
Before Meetup, we had a custom-written calendar on our web site. Our email list was on Yahoo Groups. On a good week, I would call a few people to tell them about the event. Attendance was tracked (or not) by whoever led that event, using whatever method they used.
What other platforms could we use? Small social group, open membership, limited funds and time. I'd like to avoid Facebook.