It's a great way to plant false plans. You don't have to carry them out unless the threat was never mitigated(if it wasn't taken seriously, then you can carry it out for 2x damage: part being the attack itself, the second part being that "they knew and did nothing!").

How would you know if they did know and chose not to do anything to avoid showing they did know?

Well ISIS must have some form of intelligence of their own? Although I have no idea how advanced it would be.

While not quite planning, announcements do occur. Found a large number of accounts connected to the attackers from:

https://en.wikipedia.org/wiki/Curtis_Culwell_Center_attack

when it was ongoing.

Minutes before the attack:

> #texasattack: "May Allah accept us as mujahideen."

Minutes after:

> Allahu Akbar!!!! 2 of our brothers just opened fire.

Also saw Salafists retweet an old cartoon by Charlie Hebdo of Abu Bakr al-Baghdadi hours before the Charlie Hebdo shooting occurred.

I'm divided about this issue. One the one hand, I've seen accounts that glorify the Paris attacks and link to bomb making materials, on the other hand, I've seen lots of video's from Syria and Palestine which does indeed radicalize youth, but is also not directly terrorist propaganda (a lot of this is newsworthy, but does not end up in the news, so it is valuable to share on social media). Should linking to an Anwar al-Awlaki video be grounds for a ban? Remember, these videos are on Youtube.

I like to think back 10-15 years when people had to self-host their websites. Stuff that, back then, would surely get you a knock on the door, slips through daily on social networks. I say: If you can't host it yourself, you can't rely on a third party to host it for you and protect your anonymity.

So yes, chase them underground, and apply more serious tracking to those that are acting more seriously (someone on Telegram writing about terrorism is way more serious than someone on Twitter writing about terrorism). Try to keep social media free of war propaganda. And be careful to not overdo it. America is a country where writing a Facebook post about Apple interspersed with quotes from Fight Club can get you SWATted [1].

[1] http://nyconvergence.com/2011/06/ny-man-uses-fight-club-quot...