I would be more surprised if that wouldn't be the case, not as in an actual person browsing through the latest posts and searching for what this and that individual says about something but more likely bots that scan for keywords and trigger further analysis. I'm pretty sure they do so over other platforms too, but again all automatic and on bigger platforms they probably have it already built in.

Just as an example, Facebook has been doing surveillance on its chat since at least 2012, god knows how much earlier before they acknowledge.
https://www.cnet.com/tech/services-and-software/facebook-scans-chats-and-posts-for-criminal-activity/

That aside, I would love to see how much a government agent would last while reading the P&S board, the rate at which they would succumb to brain tumors would make them cancel human surveillance.