In case you want another opinion instead of just an argument hijacking your thread...
Just kidding. Seriously, I personally hate Facebook. I have an account, but hardly ever go on. Some girls, it's the only way to contact them. But if you can do it
any other way, then do that instead.
If you add her on Facebook right away, you run the risk of her thinking you're stalking her (on some degree). I would go back to the bank, have a routine set up, and be helped by her, then let her know that actually you don't have any banking to be done today. You just came to see her. Tell her you understand that she's at work, and you don't want to distract her, bu you would like to talk with her outside of her work setting. Ask for her number, and if you don't get it, then grab a pen and paper, scribble your number on it, and slide it over to her. Tell her you know she can't just give her number to all the guys at her work that ask for it because they may be psychos, but you'll give her a chance to see that you're different. Her call. Then just smile and walk out.
If she doesn't call you after all of that, she wasn't genuinely interested, and adding her on Facebook wouldn't be a good idea anyways. If she does, then who needs Facebook?! You got her number, and probably a date as well!