My feeling is that we have entered an age that youth is not naive anymore and would happily pay $3/month (at least in the US) for a non-tracking all-round communication service.
As long as no one else they know sees...
It's not that "oh, if it'd succeed someone would have done it by now," but the attempts I've seen at this make me think that it'd be very difficult to even try.
We have yet to learne to pay for nice things on the internet.
Network effects are king.
Are you joking? When I was a broke student there was zero chance I would sign-up for a monthly subscription service.
It's a pipe dream.
Probably not, but there just might be enough, youth (or people just out of the edge of that category with contacts inside of it) that will pay $10/month for such a service plus the privilege of sponsoring 9 of their friends with free accounts. Who, would of course have the option of upgrading to paid, getting their own 9 giveaway accounts, and returning the giveaway account to their sponsor.
Which, sure, might be as little as $1/user/mo., rather than $3.
It's open source now[2], there's no advertising, and you can host your own "pod" (page) or you can use another host. https://joindiaspora.com is the main site.
Hardly anyone uses it because... hardly anyone uses it. And it's a little more technical than getting on Facebook.
[1] https://www.kickstarter.com/projects/mbs348/diaspora-the-per...
It has risen from 150k to 560k users in the last few weeks [1] with 1601 instances up.
Even if I did, I don't have what it takes to make it popular.