I ride with several groups and a few use meetup.com, while the others use an email list. Each method has its advantages, but there's no excuse if someone doesn't get it. The email message needs to be well coordinated, otherwise you end up with squabbling and too much indecision. Facebook invites seem to work better for larger event where you're trying to get a large turnout. The culture varies around the country, so what works well in one are may not somewhere else.