He doesn’t like you.
You told him that you like him on your last day of work and he didn’t say anything to that.
This would have been his opportunity to let you know that he felt the same way but he didn’t.
He agreed when you asked him if you could add him on Facebook because he didn’t want to be rude.
After all, it was your last day at work.
However, he really had no intention of having meaningful conversations with you on Facebook.
This is why the conversations that you have had with him on Facebook have been so formal and he always ends them soon after they start.
Yes, you wanted to hold on.
He just doesn’t feel the same way for you as you do for him.