Zion seems to be a moneygrab to me. They were going to use Nostr but were convinced by some very important people to migrate to ION/IdentityHub instead (because it would attract more VC money I imagine), their CEO sounds like a scammer, I don't trust them. Also I believe their tech will be shit if it ever gets finished.
Slashtags is a cool concept, but has two fatal flaws:
  1. It depends on a set of JavaScript libraries that cannot be easily ported to anywhere else, this will hinder adoption.
  2. They depend ultimately on a DHT and DHTs cannot ever work in scale (but they may appear to work on initial stages when no one would be using them).
Shame about Zion not using Nostr, I think Nostr is where it's at. In fact, I'm working on a very similar project in my spare time. I was planning to DM you about it when I have a MVP, but let me know if you're interested in taking a look before then.