Earlier this year, the Brookings Institute came out with an interesting statistic:
Thanks to Elder of Ziyon
If you calculate these numbers based on number of pro-ISIS tweeters per million Muslims with Internet access in each country, the chart looks like this:
The US numbers might be skewed because some percentage of the tweeters might be just trolls, but even so the US might want to take this threat more seriously.
This chart also indicates that Palestinians are far more radicalized than Muslims in Egypt, Saudi Arabia or Turkey. That is a subject that the media avoids studiously.