It’s unlikely that she was leading you on.
The eye contact that you experienced with this girl seems to have been more coincidental than intentional. In other words, she never seemed to go out of her way to look for you and stare.
You would both happen to be facing each other in the same class or run into each other as you were headed to class or in a parking lot and eye contact and a smile would follow. However, she never seemed to go out of her way to look in your direction.
There is a good chance that being that you liked this girl, you may have read more into the occasional moments of eye contact than was really there.
To be sure about how she really felt about you, you needed to make an aggressive move on her in either asking her out or at least telling her how you felt about her.
You never did this.
When you messaged her on social media about how she never talked to you in class, she responded by telling you that you never talked to her either.
That was a probable indication that she is the kind of girl who expects the guy to step up and go after what he wants.
You may have received delayed replies to your messages to her social media because your messages weren’t particularly fun to read. You would say hi and then proceed to talk about plans for life after university.
These conversations were just too bland. It is no wonder that her response rate would become increasingly belated with each new message that you would send.
All in all, it just appears as though you never really made an aggressive move on her and being that she is the kind of girl that would expect a guy to be more aggressive when he is interested in her, she didn’t take you seriously.