I'd avoid anything not supporting ActivityPub at this point, due to a lack of interoperability.
The main reason why most people won't embrace a certain technology (e.g., Matrix, or even Signal, or even Telegram over WhatsApp), is that their friends (or the people / groups / pages they're interested in), haven't joined the platform yet. Twitter was not popular among teenagers when I was in high school, because at the time all the young people were most active on Facebook.
So, while my preference may fall over pub Unices, traditional fora (with sections) and Newsgroups for example, the Fediverse is currently where I can get the most interaction, and this at the end of the day is what matters most for me.
By fediverse I mean any distributed social network built around:
- ActivityPub (Mastodon, Pleroma, MissKey, Pixelfed)
- Diaspora Network (Diaspora, Friendica, Hubzilla, Drupal)
Open, standardized, interoperable protocols matter. While ActivityPub/Diaspora may have their downsides and limitations, they allow people to interconnect, because they've become standards. In my experience, nobody is going to install yet another client on their PC/phone, just to communicate to a single, picky geeky friend. That's why I think that things like Movim, SSB, Retroshare, Lemmy and others haven't taken off, outside of a niche of aficionados.
My suggestion is that if you don't like Mastodon, you may try with Pleroma (more lightweight, customizable and supports a bunch of additional useful features). If you want something minimal and kiss, then have a look at Honk from @tedu. If you prefer classic social network (e.g. more like original Facebook or MySpace) over microblogging, then go for Friendica or Hubzilla.
Choosing the correct instance is fundamental, as this not only affects the sort of people and topics you come across on the local timeline, but, more importantly, determines the portion of the Fediverse that you're able to see.