The evidence I know of that hint at monitoring being effective is from the Italians, where they say they thwart terrorism by closely monitoring and infiltrating potential groups. I remember reading that the Italian police says it takes 20 people to monitor one person effectively (will need to find a link). That kind of man-power is hard to scale.

edit: link on Italian anti-terrorist efforts: https://www.theguardian.com/world/2017/jun/23/why-has-italy-...

Further debate on the SlateStarCodex subreddit: https://www.reddit.com/r/slatestarcodex/comments/6j8703/cult...