It already knows whether you are single or dating, the first school you went to and whether you like Justin Bieber or loathe David Cameron. But now Facebook, the world's biggest social networking site, is facing a storm of protest after it revealed it has discovered how to systematically control users' emotions making them feel happier or more sad at the stroke of a few computer keys.

The Californian internet giant has published details of a massive experiment in which it manipulated information posted on 689,000 users' home pages and discovered that through a process known as "emotional contagion", it had the ability to make users feel either more positively or more negatively about things without them knowing it.

Facebook doesn't know anything about me that I don't tell it.

That's a particularly naive view.