What does it mean when a guy gives a "the look" and deep gaze? What should I do if I am also interested?

I met a guy several months ago and he's a foreign student at my school. We knew each other because we were in the same group once. But we were just acquaintance. There were several times that I noticed him stared at me. But when I looked at him, he quickly turned his head away and felt embarrassed.

3 months ago, that was the last class and I walked past him. He spot me from afar and i noticed that he was so excited to see me. Then, we locked eyes on each other and smiled at each other. He looked so deep into my eyes and I could see that his pupil dilated. I could tell from his expression that he adored and wanted me at that moment. I have never been stared by a guy like this before. It felt so amazing.

But the problem is I was in a relationship at that time so i didn't approach him and initiate a conversation.

Now I am single and not engaged in any relationship. But he has left my city and gone back to his home country. I find that I still cannot forget him.

Do u think is it awkward and creepy to add him on facebook and talk to him? Is it too late to do so now? I am afraid that he has already forgotten about me.

But I really miss him. I don't want to sound creepy.