I don't use the site, but my understanding is that SS# (and photo ID) are required for members that are sufficiently popular, like the equivalent of the "blue check" on Twitter. Those members receive financial remuneration from Parler based on how many people click on their posts, so the SS# and ID are required because Parler has to report the money they give those users to the IRS as income.

I am confident YouTube also has the SS#'s of the people that are getting a cut of the ad money from that site.

So basically Parler is a site designed for people that decided that the cesspool that is Twitter is insufficiently deep and/or shit-filled, but this particular outrage is not properly worthy of outrage.

--
Nathan H.