A number of people have already shared some variant of the chart below on Twitter. It shows Google search interest in: Donald Trump (blue), Hillary Clinton (red), Bernie Sanders (yellow), Ben Carson (green), and Jeb Bush (purple).
Since announcing his run in mid June, Trump has enjoyed notably higher search interest than any of the other main contenders, mostly likely in part because of his frequent gaffes. There was a surge of interest in Clinton around the time of her announcement, which has subsequently faded. Interest in Sanders has been growing over time; it reached its local maximum around the time that his Seattle speech was interrupted by Black Lives Matter protestors.
Purely for amusement, I thought I'd compare the candidates by number of Twitter followers and number of Facebook likes. Clinton and Trump are at the head of the pack on Twitter followers, each racking up more than 4 million. This is almost certainly attributable, at least in part, to their longer-standing notoriety. In the case of Facebook likes, Trump and Carson (somewhat surprisingly) lead the way, each tallying more than 3.5 million. Clinton has noticeably fewer Facebook likes, while Bush ranks feebly on both measures.
In spite of the foregoing, Clinton is currently favourite to win, with Bush in second, Biden in third, Trump in fourth, and Sanders in fifth.