It likely has zero measurable impact on stopping terrorism.
It will have measurable impact on the millions of people who have to give up privacy to enter the US, some proportion of which is likely to be further negatively affected by having something "suspicious" in their profile. Like being idiots who once liked something offensive as a joke, or being friends with some idiot who is connected to ISIS on some social network.
Having to be careful what kind of jokes you like or articles you link reminds me exactly on how people behaved back in pre-1990 socialist era where liking or retelling a wrong joke got you in trouble with your boss, police or a landlord :/
It will have measurable impact on the millions of people who have to give up privacy to enter the US, some proportion of which is likely to be further negatively affected by having something "suspicious" in their profile. Like being idiots who once liked something offensive as a joke, or being friends with some idiot who is connected to ISIS on some social network.