I've been pondering this since my first set of students finished. How much effort should I put in to keeping in contact with my former students?
Like others have said, my students' PhD defenses were bittersweet. I was so proud of how much they accomplished, of how they had matured into capable scientists, but I miss them. And I am not the kind of PI who socializes with my group much outside of work. My first students set up my lab with me, (hopefully) spent longer in the group than anyone else because of it, and helped me actually learn how to be an academic PI. For me, it went as smoothly as I could have hoped--my senior students finished and left, and my newer students took over where needed without a hitch. But even now, it is still weird to me that I won't see my first students in the hallways or labs.
It definitely makes me wish I kept in better contact with my own advisor. My students have a very different relationship with me than the relationship I had with ProdigalAdvisor, but now I think it would have been nice to send an email more than every few years. So far I have been in regular contact with all of my former grad students except one, and I hope this continues. I really want to know what they are up to and how things are going.