There are things that can easily be detected and cause you to have issues with a clearance. For example, if you get arrested or have other significant legal issues, someone will probably find out. Also, if you stop paying your bills long enough to have severe credit issues, someone will likely find out about that too.

For social media activity, someone would likely have to report you for anyone to take action.

I for one think its pretty reasonable, it sucks working for the government enough already, if we put people under 24x7 surveillance just for trying to serve their country they will go from having very few talented/ethical people working for them to absolutely none.