It's been five years since Beyonce's ubiquitous female anthem "Single Ladies" came out, but here in San Francisco we're still working our backsides to solidify its legacy. So much so that devotee dancers are celebrating B's birthday in the streets with a four-song flash mob on Sept. 7. Dance classes started in July and continue through the first week of September, so that those who'd like to participate can learn how to 迭un the World with the rest of Sasha Fierce's army. "I-don't-need-you" power pop is always going to resonate with the masses; Beyonce and the other two long-legged ladies of Destiny's Child had already proved that with megahits "Independent Women" and "Survivor,"迫but when Beyonce served up the fiercest riff on the song "Mexican Breakfast" (featuring the Bob Fosse choreography that inspired "Single Ladies"), her dancing shot to another level. YouTube toddlers, Justin Timberlake, and Joe Jonas followed. Catch the last class and join the mob.