I'd think that having her as a facebook friend could lead to breaking confidentiality (of other patients) or having her favour you over her other patients, then also give therapy to a friend. I just see a bunch of ethical/legal problems with that, but if those didn't exist, then sure, go for it. You could also talk to the T about it in a session about adding them and seeing their response.

However, I'd assume that what happens in the session stays in the session (unless you're an immediate threat). Or just simply let it be part of her personal life and let it be part of your personal life.

I haven't a clue whether my T has facebook simply because I don't have one so her having one wouldn't concern me at all.