How did it profit from the Usenet archives? Genuinely curious.
Dejanews was the seed material for Google Groups, any profit derived from that (ads) was from content posted to Usenet by people who never intended for it to be used for that.
From G side they gained users interacting with 50K+ topics and occasionally posting views, sentiment etc. (and likely all Deja News historic interactions)
In addition to the search history, email content, geo location etc. G have for many people