Try to mention something in her profile.
Or try "wanna go and sing around a fire? We can act like hippies again and reminisce about the good ol days.
How ya been sunflower?

"
The idea is your roleplaying with her and making her recall good times that have never happened before. You should modify this until your comfortable with it.
You could also try commenting on one of her pics "you remind me of my weird ex

" this shows your preselected and opens her up for conversation. If she never replies you could message her directly.
The best way is to be a geniunely interesting person and have cool pics and cool status updates. This would give her many opportunities for her to open
you up. But this is more of a long-term plan.
but yeah, basically you have to open her up. But your best bet is to get her phone number in person if at all possible, remember that next time.