The Canadian Friends of Somalia, a non-profit organization serving the Somali community of Canada, hosted a conference in Ottawa in December 2010 dealing with Somali youth radicalization and support for al-Shabaab. It subsequently released a report based on the conference titled Youth Radicalization: Somali Identity and Support for Al-Shabaab in the U.K., the U.S., and Canada. At the time of the report, an estimated 40 American-Somalis and 20 Canadian-Somalis were believed to have joined al-Shabaab. I participated in the conference.
You can request a copy of the 16-page report here.