I guess some people use FaceBook for different reasons but to be honest I think its slightly strange that she's asked me.
I really only have FB for close friends, including a lot who have moved overseas and old uni mates who I don't see often. Its great for keeping in touch with them.
I work as a midwife and someone who I met through work, for a couple of hours has requested me as a friend. Its not even as if I was her community m/w and we built up a relationship over a period of months, I looked after her in labour. I don't know her, she seemed nice but I don't want to be her mate. But likewise I don't want to offend her by declining her.