In general, Al Shabab is a regional threat to African affairs. Most of its rank and file members are concerned about local issues centered around Somalia, Uganda and Kenya. Even though Al Shabab's main focus resides in the continent of Africa, top leaders have international ties with Al-Qaeda.
Recently, Al Shabab has splintered into two group: a 'foreign legion' and a 'national legion'. Since the 'foreign legion' wing of Al Shabab has aligned itself with Al-Qaeda, there exists a potential threat to American soil. Even if that potential is weak, the possibility remains and should be taken into consideration.
Al Shabab is a recognized terrorist organization. They have the capabilities to attack targets in Africa, which they have even wanted to recruit Americans. This poses significant and dangerous risks for American citizens. It is vital that we protect our security by completely eliminating their ability to harm innocent lives here and abroad.
Al Shabab may not be as strong as Al Quedia but they are still a force to be concerned of, I think as long as hostility remains between Al Shabab and the United States that they may decide to one day launch a attack on the United States in some way.
I believe it is possible for Al Shabab to be a threat to American Soil, although there's no evident proof that they are a direct threat. I believe this is the type of stuff our investigating agencies should zone in on, rather than trying to track everyone in the world.
Realistically, I do not see many people or countries as a threat to American soil. We have military bases everywhere, the most armed civilian population on the planet, and intelligence agencies that have been thwarting attacks for centuries now. It's not a concern to me at all. I'm not paranoid.