I have a colleague who's about to retire. I really enjoyed working with her, but we were never really close enough to see each other outside of work hours. And she could be my mother.
I wrote her a nice personalized note in the happy retirement card. And she keeps telling me she wants to keep in touch, add me on Facebook and have my personal e-mail.
Maybe I'm different, but I don't get that attached to my colleagues. There are some I like more than others, but they're still people from work. I don't feel like keeping in touch?