Also, what if she was miffed, then. But not so miffed that it was really big thing. She vented to her mom; her mom blew it out of proportion.
Later, she thought it wasn't such a big deal, and now that she's vented, she can see that she wasn't really all that reasonable, and so she decides it's not something to pursue, and in fact she has -forgotten- it.
And now it's going to become an "issue" between the two of you? Let it lie. Give her room to get over it; I would bet that she already has.
Personally, I don't see much of a downside in testing to see if there's a chance at some level of friendliness between you. What's the worst? That she won't respond, or will but then fades out.
So I vote for sending a note directly to her on Facebook, and see what develops.
Or, just click "like" a few times, and chime in w/ a small note here and there, and see what happens. Just as you would in making a new friend.
Because that's what you're trying to do, really. You're starting fresh, really. It's just that there would be memories that are a bonus attachment.