I can identify with those of you who had work friends who "disappeared" after you no longer worked together.
Here are two stories about the best job I ever had and the "friends" I made there:
1. One was my boss. She was everything you ever wanted in a boss--kind, encouraging, understanding, humorous...etc. She had a great relationship with our team (we were journalists at a newspaper, and divided into teams). Used to put little Hershey kisses and stuff like that on our keyboards to surprise us. After she took another job in another city, we never heard a word from her. People in the newsroom kept asking us, "So....what do you hear from Mary?" It was almost embarrassing to say that we'd heard zilch. I talked to my spouse about it. His take on it was, "She really WAS a good manager...but not your friend. After all, she got you to do what she wanted you to do, and you all liked coming to work." In a sense, I felt like I'd been "had."
2. Same job: I did have a real friend on the same team (Susie), who became my manager after Mary left. This was fine--our relationship did not change at all. She never needed to "be the boss" with me--we were more like a married couple. However, after I left the paper, the only time I saw her was 1) at her wedding and 2) once for lunch. Oh, and once I ran into her at the OB/GYN. She is not much of an online person, and we never talked much on the phone.
Said workplace had an employee reunion last year. I did not attend. One of the organizers kept trying to entice me to come by telling me Susie was going to be there. I was tempted (even though I was undergoing some social anxiety issues). Then I thought, Hey, Susie hasn't bothered to email or call me in years. I'm not going just because she's going to be there.