Another Oxfordian here (well, Abingdonian to be precise...):
I have a FB account, but do not consider myself to be an active FB user, and wouldn't really want to go down that route. Happy with any other reasonable alternative, e.g. a blog that one could comment on (without having to jump through too many hoops - some blogs are better than others in that respect).
Certainly would not want to restrict the Oxford Meetup to the SSiW "bubble" - on the other hand, it would be good to try to make sure every SSiW member who could reasonably get to Oxford becomes aware of the Meetups, supposing they should wish to attend.
I'm actually quite keen to make it more often than once a month actually. Not really quite enough. Not that everyone would need to feel they had to attend each meeting, but there would be more meetings in a year, and maybe more people would have the chance of attending. On a monthly basis, if you can't get to one meeting for any reason, then that's no meetup for 2 months - quite a big gap.
Something to think about anyway. I have actually privately invited a non-SSiW person to the Oxford meetup. So far he has not "bitten", but I still have hopes. He's quite an interesting character, and I suspect we could all learn from him.