I got myself a Twitter account just four weeks ago. It seems that my fortunes have taken a turn for the worse, having me barely edge out al-Shabaab, Islamist terrorists’ recent cause célèbre, which logged its first tweet on December 7. Al-Shabaab, or “The Youth,” is a militant Islamist group with ties to al-Qaeda that has laid siege to much of southern Somalia since 2006. The group is persistently fighting (and winning) an insurgency against the United Nations-backed Transitional Federal Government, which is contained in Somalia’s capital, Mogadishu. Still more pressingly, al-Shabaab has mounted attacks against African Union Mission in...