I tend to use LinkedIn for that. It's fairly asynchronous and not too "in your face." I used to roll my eyes at their anniversary reminders, but then I realized

I tend to use LinkedIn for that. It's fairly asynchronous and not too "in your face." I used to roll my eyes at their anniversary reminders, but then I realized it was a great opportunity to find out how someone was doing and just do a brief keep in touch kinda thing.