For keeping in touch with ex-coworkers, I use a mish-mash of different systems, mainly Facebook, IRC, Discord, and email. Whatever the contact uses, that they are willing to share with me.

Ten years ago LinkedIn was the preferable system for managing contacts, and I would still appreciate something better, but with how LinkedIn has fallen there just isn't anything viable. A better replacement would need to be used ubiquitously, and right now there is no such system.