The arrested persons total 30, a number I imagine is large enough to worry federal authorities. There is no way for me to know how many supporters of Al Shabab have so far escaped capture, but it seems likely that there will be further arrests.
Media reports – and the charges themselves – typically frame the issue as terrorism, which misses part of the picture. It is worth asking how al Shabab’s supporters in the US see their own activities. My guess is that many of them see terrorism not as goal in and of itself, but as part of war. Al Shabab’s attack in Uganda this summer was unquestionably an act of international terrorism, but the movement’s primary goal seems to be conquering southern Somalia. Some of al Shabab’s supporters in the US probably do not even identify with the goals of transnational terrorist movements like Al Qaeda – and in fact so far Al Shabab’s US supporters have concentrated on helping the movement with its activities in Somalia rather than, say, plotting attacks outside of the Horn.