ICYMI, @stux did a post highlighting how Mastodon can be used as a private intranet just for you and your friends.
This is thanks to Mastodon's optional "limited federation mode" which uses allowlists instead of blocklists. This mode blocks all servers by default and only federates with servers you list. If your allowlist is blank, then it doesn't federate at all.
If you're interested in using this mode, there's more info at:
🌱 https://fedi.tips/creating-an-isolated-server