Thursday night’s abhorrent online vigilantism — in which Reddit and Twitter users seized upon police radio chatter to accuse a missing (and completely innocent) Brown University student of bombing the Boston Marathon — reminded us of one of the most under-acknowledged facts of the internet: that beyond the sleek, profitable edifices of Web 2.0 there remains the humming, virtual presence of an online crowd that is restive, unpredictable, and hungry for a cause.