And it's not knowing where they are or what's happened that's the worst.

If only the person was able or willing to post an entry or send a PM saying, "I'm exclusively on Bluesky now" or "I've got a big-time public-facing job, so I'm purging my fannish identity"--I mean, you would still be sad to see them leave the community, but at least you'd know that the friend was doing well, and you might have the option of keeping up with them on the other platform or IRL.