We often hear of "flash mobs" gathering for less than decent purposes. Think of the recent riots in London. However, these new and ad hoc social gatherings sparked and organized by social media can also serve very salutary purposes, as shown in this video. Here's how Dictionary.com defines "flash mob": "a large group of people mobilized by social media to meet in a public place for the purpose of doing an unusual or entertaining activity of short duration."
No doubt, this video is intended to advertise the Copenhagen Philharmonic Orchestra. But what a great joy for those in the train terminal who were treated to a free, one-piece concert! Just look at the expressions on the faces of the onlookers.