The US Republican and Democratic conventions have traditionally targeted television viewers above all - but this year's will see an unprecedented effort to reach voters via social media.

In the last presidential election four years ago, Facebook, Twitter and other platforms were not yet the ubiquitous force they are now - even if Barack Obama was among the first to use online tools in a major way for electoral ends.

It was arguably not until the US mid-term elections in 2010 that social media, including YouTube and Flickr, really took off as key tools in the politicians' communications armory.