I think a platform would need to fit a few criteria:
- Open to the general public and 'free of charge' (no subscription, fees, or paywall)
- Driven by user content
- Any content moderation is not done centrally by the company (i.e. Reddit allows anyone to run their own 'reddit' and those 'reddit' mods can moderate as they please, but Reddit itself is not moderating content beyond what is legally required.
- Size. As soon as you're big enough to be a 'public square' and enjoy a natural monopoly.
If you fit all of the above criteria, you are defacto a platform.
Smaller companies that want 230 Section protection should be allowed to self-declare as a platform if they meet all the criteria except size.
The legislators can figure out the rest of the details.