I tested Threads. In my experience, this is how Threads compares to Mastodon:
— The algorithm decides if your content is visible to others. Some content gets zero exposure for reasons which are not transparent.
— Sharing links to other platforms makes the algorithm suppress your content's visibility.
— Slower to use.
— Only one app allowed, and it does not support iPads and other tablets.
— Ads are coming in early 2025.
— You cannot run your own server.
— Your data is mined by Zuckerberg.