I would think virtually all schools at least monitor the chatter about their schools online. For college admissions CC is an obvious place to pop into and see what is being said. I would expect schools like Harvard, with lots of posts related to them on CC and a pretty clear ranking/prestige level where a few negative comments won’t hurt them, to not read every single post but they might look for a general sense. But for less discussed schools it isn’t that hard. I can pretty easily pop in occasionally and check on the latest chatter on my kids’ schools or my own schools. Colleges would be silly not to in case there is a negative story being spread, misinformation, chatter about some sort of issue with the application process (tours, info sessions, good/bad communications etc).
I highly doubt they analyze kids to protect yield (with the possibly exception of cherry picking kids who stand out positively off the waitlist). However, I wouldn’t be at all surprised if they figured out who certain posters were in real life. I’d expect mostly if someone clearly showed themselves to be someone they wouldn’t want at their school (displaying some sort of very negative personality trait, admitting to lying or cheating etc) then they might take a few minutes to try to ID them. I don’t think it would be hard in many cases. But it would only worth the effort if what they read made an alarm go off. I also don’t think they would go looking for people to identify and reject but if in a general checking in and monitoring the chatter about their school they saw some comment (or more likely repeated communication) that jumped out at them enough that they’d want to not accept that student then try to identify them.
On the occasional identifying of a student I am speculating but I’d be shocked if most schools don’t at least occasionally monitor the chatter about them either themselves or thru some sort outsourced service.
It seems and easy job for a student working in admissions.