At least, to me it's odd.
So here's the report, while it's fresh in my mind. Went out to a local band's show (they're decent and usually attract cute artsy chicks, which I like) at a nearby bar. Ran into a guy I knew and his group - two slightly older women, an older guy and a couple guys around my age (late 20s or so). Hung out with them, spending time with the men and also making a few rounds, bringing others into the conversation, etc.
At some point this girl who is in a band other than the one playing becomes involved in the conversation. Now, she was not a target because I had not actually even noticed her previously but I liked what I saw. So I did some playful negging, made some jokes, had her laughing, we talked about playing an instrument we have in common, she insists I come to her next show, I go to make plans to see her again after my finals (grad school) are over. That'll be a couple weeks out because they start this week and I will NOT have any time free once they start. So I mention how we need to be in touch, reach for (don't pull out) my phone and ask how that can happen and she explains she never checks her phone at all and so facebook is the best way to get in touch with her.
I have no idea about game on facebook, whatsoever. I don't know how common this is because I've never had problems getting numbers in my entire life so I'm a bit bewildered by the "not going to give you my # because I never check it but I will respond to you on facebook" thing. Maybe I'm overthinking it.