VoluntaryNet is:
Decentralized, not federated.
Browser based, not a native app.
A platform, not just a particular app.
Apps share: identity, discovery, relay, and secure messaging.
Discovery is decentralized (anyone can add a rendezvous server to the network).

Federated systems (such as email, Diaspora, Mastodon) have servers with owners who effectively own your identity and can censor your messages. In decentralized systems, you own your identies and messages are effectively uncensorable.

A bit, but it's different in important ways. Mastodon is a federated Twitter-like native app. Voluntary is an in-browser decentralized platform on which a Twitter-like app is provided as an example.

They are different classes of things but complementary. Beaker Browser is a type of web browser that gets files from a decentralized file store (IPFS). Voluntary.net is a single page web app that could be run inside most web browsers, including Beaker Browser, or even directly from a file on your desktop.

Zeronet, like Beaker Browser, is focused on websites but (like Voluntary) adds support for identity management. Zeronet and Beaker require special native apps. Voluntary supports identity management in conjunction with a common messaging system, and works within most browsers.

No. Steemit is centralized ad-based, highly censored social media platform. Voluntary is decentralized, add free, and has no trusted parties which can censored content.

No. Blockchains are useful for distributed ledgers but create scaling, latency, bandwidth, and other issues for large systems (such as messaging systems) which don't require everyone to agree on a common global state.

As you can build centralized apps like Facebook on top of a mesh network, a mesh network in itself doesn't make for a top-to-bottom decentralized communications system. So we also need a decentralized identity and messaging layer, and true decentralized apps on top, in order to have a top-to-bottom decentralized communications/applications system. VoluntaryNet is one solution for those top two layers.

No. This is just an open source project and the goal is for everyone to run it localy in their web browser and eventually not use the voluntary.app site at all. All keys and data are stored client side. There's no ads, tokens, paywalls, etc. There may be people that want to offer always-on message relay servers for a price, but as long as you have enough friends in your network, you shouldn't need one.