@ILoveWoodstar,
I joined Facebook when my nephews friended me. It seemed harmless, but as time wears on I am bombarded by friend requests, I've had messages from folks accepting my friend request when no friend request was sent. I've unfriended people, only to see them reemerge accepting my friend request. I find Facebook a pale imitation of real life, so I've set my account to be as private as possible.
I would discourage you from relying on Facebook for actual friendship. But I'm not a Facebook fan.
I don't think your 3rd grade buddy is annoyed you didn't recognize her during your exam. Most people are so stressed when they take exams the only thing they notice is their own heartbeat. Plus, she didn't indicate that she recognized you, I'm pretty sure you were not offended.
Another thing, if she was bullied, it might make any encounter with elementary school classmates painful. I'm not saying you bullied her, just that she doesn't have good memories of school.