FBI wants to monitor Facebook and Instagram for domestic threats in real time
lite.cnn.io
lite.cnn.io
(The former being protected by federal wiretapping laws that have been around a looooong time)
Additionally, something that is different is the scale. In the US, an activity may be legal if done once but if done millions of times it may be illegal. Here, there are many times more people plotting, discussing, and celebrating terrorist events online than there are on traditional phone calls. Having phone calls be traditionally 1-1 and private internet chats be 1-many, even 1-1000000 or 1 to 7000000000 if public, is a totally different beast. I'm not sure there is any analogous previous laws, I think we need new laws.
For example, the military could regulate the militias in exchange for access to military equipment and a promised to join in officially declared wars under federal command. I honestly don't know the particulars (i.e. what militias would accept), but more formal ties with the government could ease tensions with these groups that just want to defend the America they believe in. Some groups obviously wouldn't accept, but that just makes it easier to tell who is likely to cause problems.
I agree though, I'm not worried about mosques, I'm worried about extremists, and extremists come from a all manner of backgrounds.