Mastodon

I think the trickiest part is finding people on other instances and needing to copy/paste their links in your home instance's search bar before you can follow or reblog, especially if you're following a link someone's shared elsewhere. It's a small nuisance, but it adds up over time, and it's already more work than most social media consumers want to bother with. For Mastodon to truly take off, that needs to be automated or hidden, because most people are going to give up before they even get an explanation.