Most popular platforms are tracking and spying on you. My friends and I also believe Slack private DMs are compromised as we often times see ads directly pertaining to oddball discussions we don't have outside of Slack.
Most people here probably know this already, but you can minimize some of this by using privacy browser extensions [1], containerized browsing [2], a good VPN [3], and/or Pihole [4].
1: https://duckduckgo.com/compare-privacy?tab=extensions
2: https://support.mozilla.org/en-US/kb/how-use-firefox-contain...
3: https://docs.google.com/spreadsheets/d/1CRtEQzSVE59jj5ROKZlt... (*do your research, e.g. NordVPN creeps me out with aggressive advertising practices even though they're highly rated)