Thank you for all your thoughtful responses to my question about how to announce on commercial platforms that you are leaving.
I may keep my accounts deactivated… and EMAIL the people I want to keep in touch with instead. One big reason:
The opaque algorithms of Big Tech may hide my post, esp if it contains keywords like “leaving” and “find me here instead” with a link to an external site.
In my email, I will definitely boost the Fediverse and volunteer to help friends set up accounts here 
@_elena Email providers and commercial social network companies are mostly disjoint sets. Furthermore, email transport and delivery is mostly free of opaque algorithms that could hide messages, except for spam/abuse filtering, which for most end users is opaque and out of their control. For the time being, email is reliable for the use-case you described. I'm learning not to take anything for granted. I'm happy there's a way to communicate without interference.
@_elena I have no doubts about the validity of many user‘s experience, but I can report that I have a note in the profile of my IG account that clearly reads that I am continuing on the Fediverse. It also contains a link to my website which is routed directly to my new Pixelfed account. Maybe my IG presence with about 300 followers is too tiny for them to care about. Or their bots just crawl through posts but not through profiles and settings?
"Write a typical LinkedIn post telling that I'm leaving LinkedIn for Mastodon without mentioning typical words like "leaving" so that the LinkedIn algorithm doesn't auto-hide my post " 
@hikingdude 
I am EXCITED to share with you today that I will be pursuing a new challenge 
, building up a new circle of my 
awesome followers on a new revolutionary platform 
.
"Wherever you go, there you are" -- my mentor, probably
