Of course, that's if you only count incidents with 4+ fatalities as a mass shooting. If 10 people get shot but no one dies, is that not a mass shooting? I mean, by that definition, the attack on the Colorado Planned Parenthood facility isn't a mass shooting because only 3 people died.
True enough, there's a lot of kibitzing over what constitutes a "mass" shooting, exactly. 3 people? 4? more? Number dead, or wounded/both?
Anyway...
The more I read about San Bernardino, the more it looks carefully planned and executed, not a spur-of-the-moment thing. Three guys, equipped with rifles, wearing ski masks and tactical vests. They burst into a heavily trafficked building, start shooting, plant a bomb, then quickly escape in a black SUV. They didn't stick around to engage law enforcement, they didn't issue a statement, there's no obvious political angle, and nobody is claiming responsibility. It seems unlikely (or at least uncommon) that someone wanting to kill a certain person specifically would enlist the help of 2 other people, so it seems more likely the location was the target rather than any specific person there? But whatever it turns out to be, this is clearly not a crime of passion or insanity. This was a calculated act of malevolence.