Hubzilla - Osada - Zap are one family.
The brains behind them all is the same there are other developers involved in all three.
Each has a fairly defined use-case, but (once the transition to Zot6 in Hubzilla is complete) - all three will be able to intercommunicate with each other.
Hubzilla is the most established product. It should be considered more of an "application platform" or maybe a "publication platform" (kind of like what WordPress has become since it now has plugins that allow you to do almost anything - it's more than a Content Management System). "Social Media" on Hubzilla was one of the first use-cases adopted by a large number of people - and so many think it is primarily a social media platform. It is not. It is a privacy and security aware application platform that can support a huge number of use cases through the plugin/addon architecture which is lean and efficient.
Osada is more intended for end users interested specifically in Social Media applications that want to be able to communicate with ActivityPub but also have SOME of the privacy and security capabilities offered by Hubzilla. ActivityPub has a larger user base but is fundamentally incompatible with certain privacy and security models. So, anything related to ActivityPub is going to be a compromise. Osada is a social media platform that brings as much of the privacy and security features of Hubzilla as is possible while still providing integration with ActivityPub.
Zap is a social media platform for those who are privacy and security minded and are not willing to compromise privacy and security just because "everybody is doing it." It has the full range of privacy, security, and censorship resistant features inherent in the Zot protocol as Hubzilla (without Federation addons).
If you are an "end-user" of social media and want to be able to seamlessly communicate with ActivityPub, you want to use Osada. If privacy, security, and censorship resistance is important to you and you're willing to give up access to other protocols in order to maintain those things, you probably want Zap. If you are primarily interested in providing content and data to others on an extensible and robust manner with privacy, security, and with built in ability to provide redundancy in case of failure or as a censorship resistance feature, you want to consider Hubzilla.
As you can see, the choice between Hubzilla, Osada, and Zap is a USER centric question about what features you want or need.
I am a gplus refugee attempting to find something decentralized and maybe more featurefull than that. ...but maybe having access to the people on activitypub.
Given that, I would suggest Osada - If you have enough influence to pull most of those you want to communicate with over with you, Zap may be an option as well if you are all interested in privacy and security. There should be public Zap installations coming online shortly.
Full disclosure: I run the UseZot Osada instance ( https://osada.usezot.net
) and we intend to have a Zap server online before the New Year as well. I also run Zot.Social ( https://zot.social
) which is Hubzilla. However, Zot.Social will not be enabling the federation modules to talk other protocols. It will have some advanced discussion features and be more of a platform for "content providers" than end user social media.
Once the Zot6 implementation on Hubzilla is complete, all of these projects will be able to intercommunicate with users on each of the others with fine-grained access control.