Shares in Facebook slumped by almost five percent on Wall Street on Monday after its services – including Instagram and WhatsApp – were knocked offline for several hours by a technical problem. Technology shares in general saw a selloff too, including Facebook's rivals like Twitter. Around $6 billion was wiped off the personal fortune of CEO Mark Zuckerberg. Also today, oil prices have jumped after the latest decision by OPEC and its allies not to increase supplies at a faster pace.