If you skip the technobabble and politics about free (as in freedom), what’s left? If it’s just a platform that feels more complicated to sign up, because you have to learn about instances and it’s not clear which one you want, plus your friends aren’t there, plus it’s just 45k users total instead of a lot…?

The complication arises by making the mistake of pointing people to the backend, and the backends confusing matters by presenting themselves as platforms like existing corporate platforms. As noted, you reduce that by inviting them to join or browse your respective instance (or if you’re self-hosting, to whichever open instance you think is amenable).

You’re right though that some positive thing would help, and that’s really down to whatever positive thing you found and want to share with others about these spaces. For me it’s as simple as them being open and ad-free. I’m reminded of it every time I find myself trying to browse enclosures without having an account and they simply won’t allow me to browse much before prompting me to sign up or subscribe to view more.

In a way that’s kind of the irony of the fediverse, a major feature is that you don’t have to sign up at all in many(most?) cases.