Did social media create Egypt's revolution?

Hosni Mubarak has stepped down as president of Egypt, after weeks of protests by anti-government demonstrators in Cairo and other cities.

Social media has played a crucial role in the unrest in Egypt, with many of the protests organised through Facebook.

The Egyptian government reacted quickly by blocking social media sites but this act of censorship was spectacularly unsuccessful.

The BBC spoke to new media expert Walter Armbrust.