They’ll never have the full Mastodon functionality because they’re Facebook. It’s always going to be a one way proposition where Masto can see them but they can’t see us. It’s honestly kind of like being a creepy Peeping Tom.
Assuming, of course, that your instance doesn’t block Threads. Many (most?) do. Some even block second-degree connections.
President Biden is now posting into the fediverse (www.theverge.com)
Biden’s Threads posts can show up in Mastodon clients.