It won't though, this isn't going to be like the Digg exodus.
Digg and Reddit were basically like-for-like, the friction in moving over was non-existent, because the focal point of both sites was the interesting links that people would post and discuss in the comments.
Twitter is different because the 2010s social networks did everything to kill external links, and turn the focus on the 'personalities' on the platform.
Also trying to go from Twitter to Mastodon is going to be like what beginners experience trying to set up a crypto wallet. Terrible UX, especially with dialog prompts with inscrutable messages about 'instances' that are meaningless to people coming from a walled garden.