Ease of use, I disagree. 99% of people already go to the one big instance of whatever service, whether it be Mastodon.social, Piefed.social, Lemmy.world, whatever; you type in your email and a password, maybe click a verification link in your email, and bam, you're set. Compare that to Facebook where you do that plus they'll ask you to set security questions or 2 factor auth, maybe even make you take a selfie or a picture of your driver's license--signing up in the Fediverse really ain't that hard. It's arguably easier! I'm not sure why people still say otherwise. Maybe to ease what little guilt they might feel about supporting monstrous corporations? But the truth is most people don't want to sign up for anything, period. IF, and that is a BIG IF, they go for a new service, they'll want to click the button that says "sign up using my Google account" and be done with it.
The real problem is the same one every gaming service that has ever tried to compete with Steam has: the default is already set. This is the type of problem that Epic hasn't been able to solve by offering tons of free games, nor GOG with its lack of DRM, nor Microsoft with its $1 Gamepass. Those services survive, sure, but the position of "top dog" has never been in doubt.
You might point to Bluesky as a success story, but it only got the audience it did because they ran an effective ad campaign with the original founder of Twitter, right as Twitter itself enshittified to a nigh unbelievable degree. It lost core features like reliable checkmark verification and blocking other users, and the new owner was making Nazi salutes live on national TV. And Bluesky still haven't truly managed to take the throne! Twitter is still the biggest service of its kind! Meanwhile the myth that Mastodon is any harder to join than Bluesky or Twitter is still pervasive, so people don't even give it a chance.
So, publicity, sure, that's a real problem we have, but that's a paradox that's damn near impossible to solve: people don't want to move away from where everyone is already signed up because that's where the most content is, and the most content is there because that's where everyone is already signed up. All we can do in the meantime is continue to exist as we have been.
My cope is that I don't want to interact with people who are so incapable of signing up for new services anyway. Whatever their reasons for their impatience, whether it be their nature, or if they have a stressful, time consuming job, or if they don't know how it works and just use whatever their grandkids signed them up for--whatever the reason, it's going to extend to their ability to disseminate what's happening on the platform, too. Those people are (in general) way more susceptible to misinformation, to spreading it themselves, and to generally being a toxic online presence wherever they go. So fuck 'em, we don't need 'em!
I was trying to preempt the argument that picking an instance is confusing and is the big "ease of use" barrier. Which it probably would be, except that most people don't bother with that: they just sign up with the big one. I don't mean to imply that's a good thing, I agree that a stronger system to distribute users among instances would be cool, but I don't think it would make a difference in the number of people joining in the first place. I think that network effect is easily the biggest hurdle, and there's not much we can do about it systematically outside of keeping the network we do have alive and making incremental improvements. I'm not sure there's any killer feature we could invent that would really swing the needle. Bigger, better funded orgs than us have tried.
Otherwise, it's just about us as individuals, doing what we can to push the people in our lives away from corporate social media.