Mastodon Social and Signal seem to cover these use cases pretty well. Both already have sizable userbases too.
Signal is centralized.
Are they better than Facebook and Facebook Messenger? Yes, of course.
(I use Mastodon though, the community is basically all Linux users/sysadmins/developers/etc which is great.)
I see some major forces for counterpressure to centralization-gravity on Mastodon:
(1) Mastodon starts with a homogeneous software base (ironically enough) giving “standard” back and front ends—which is then AGPL, meaning that instance owners can and do produce forks, but the forks cannot be held as Secret SaaS for lock-in since anyone who can access the instance can copy the source code from them. (It's probably possible to produce proprietary products that interop through ActivityPub, but this is harder—and importantly, you'll be the odd node out.)
(2) The idea of a “local culture” is prevalent and visible, with differences in content policy, theming, and contents of local/federated timelines. The forks from (1) can amplify this by acting like per-instance customizations with “partial stickiness”. Being on casual-contact terms with your instance administrator is also not uncommon and sometimes encouraged, whereas email is much more impersonal.
(3) The pervasive visibility of (2), mostly from instances doing their own CSS/text tweaks, provides a bridge to the “me but in a different social context” cues that are often missing in centralized social networks, which a number of people are starting to pay more attention to the need for. This while the interfaces remain basically isomorphic enough that there isn't a major UI/model-learning cost to jumping between instances.
(4) Conversely to (3), for users that want efficient access, third-party clients that support multiple accounts at once are very common. But a Web browser also counts as an intermediate form of multi-account client—so long as your accounts are on different instances, which is thus encouraged for convenience!