Do I friend or do I go?

I have a new coworker who started on Monday. She's super nice.
My conundrum: She friend requested me tonight.
And I like not having any of my coworkers having access to my Facebook. I mean, I really enjoy having none of my coworkers being my Facebook friends. I feel more free to air my mind and all that when I don't have anyone who may be judging me for it at work the next day. No joke, I talk about pretty much everything, and check-in pretty much everywhere, on Facebook. I don't know if I want someone that I see five days a week knowing what I do on the weekends, too.
Instagram is a totally different animal, since I don't really live my life on it, and most of it is dog and kid pictures, and no one can know too much about me based on the Instagram pics. I already have a coworker following me on Instagram. But Facebook? That's a lot of my mind, body and soul up there. My coworkers don't need to know my political leanings, or what my passions and causes are. When I was training with Trainer Dan, he told me that we can't be "friends" until we're not working together. And I took that to heart, really. I understand that. It makes total sense.
So here's the difficulty: Do I just explain that to her? Or do I ignore the request? I'm not totally comfortable exposing myself like that in the workplace. But I'm not comfortable being standoffish and considered rude, either.
And she's super nice.