I think using social media to gather intel is entirely reasonable. Given the number of people who have committed mass shootings and other acts of domestic terrorism after being radicalized online, they would be negligent to not be proactively looking at the internet.
Your whole argument is just "sure this is reasonable now but what if there were 8000 agents online and they could extraordinary rendition you". 80 agents for the whole country is not absurd. That's less than 2 per state.