I wonder how they did it before Facebook?
That's missing the point. What they're saying is that people they're trying to invite now only use Facebook. And don't necessarily respond to other options. I'm sure you could call every single person (if you even have their number since some people don't bother sharing that as much these days) but even then the people aren't getting Facebook reminders so they might not remember it.
News flash: amateur astronomers are nerds, as are geologists and peleontologists. You can hardly do any of thet that without going outside.
Better yet, Google Earth and Google Sky...
The generation of random numbers is too important to be left to chance.