I think the idea is no ads -> nothing to target -> no need/value for personal data. Sure, maybe Cambridge Analytica can scrape Scuttlebutt or Mastodon but then what can they do with that data?
Aside from the likely ability to connect it to other sources of data so they can show better ads on those platforms, there's regular market analysis - it's worth knowing that teenagers are increasingly interested in X even if you can't tie that back to individual teenagers.
