What problem are these "alternatives" addressing? If I'm concerned about privacy and data sharing, Twitter, Snapchat, and Tumblr don't seem like good candidates.
Diaspora and Mastodon are good alternatives (but are ironically not on this list). Both are federated solutions that let you set up your own instances, or use public ones.
It’s hard to see if any of these self-hosted or decentralized solution would ever take off. For a social app, you need to get your friends and also public figures to use the same thing for it to be worth using at all. If it is more complicated to use than Facebook or similar, it will probably never be adopted by general public. And if the general public actually got into the same app, it would then become a new target for attacks.
they're not decentralised in the sense that you seem to be implying. They're all federated, meaning that your data is situated on your own or a server of your choice, while the posts and users are still visible globally. You are not limited to only interact with people on your own instance, this is very much the point.
Diaspora allows for quite easy interaction with the mainstream social media apps as well. Also I haven't used diaspora much so I can't speak from experience there, but creating a mastodon account is no more complicated than signing up for twitter.