Well it sounds as though it was a spur of the moment decision to go. It's a tradition for the one family, but they only decided to ask their houseguests that morning. I wouldn't have necessarily thought to invite everyone else who might possibly be interested either.
Also if she wanted to do something with the group of cousins staying at the house, couldn't she have organized something herself? Or at least asked, "Hey, what are you all doing on Saturday? I'd love to see more of you...why don't we go do something?" She could also have gathered up her family and gone to the Christmas tree farm herself, hoping to run into them if nobody picked up the phone. It's a public place, right? When I'm out and about with stuff going on around me, I don't always hear my phone ringing.
As for Facebook, sometimes I've posted things and then got a comment, "Hey, that looks like fun! Please let me know if you go again!" I think that's OK, as I might not otherwise have thought of that person or known they'd be interested. Can't always think of everyone I know every single time.
I know a Christine too. Gets highly offended when she sees pictures of people doing things together on Facebook, but never bothers to contact people herself to initiate outings.