The Mail on Sunday has estimated that the Conservative Party paid in the region of £7,500 for an advertising campaign that doubled the number of ‘likes’ on the PM’s Facebook page in one month to 127,000.
Labour MP Sheila Gilmore accused him of "buying popularity" and "paying for fake Facebook friends".
However, Ogilvy Public Relations digital and content managing partner Michael Darragh said campaigns like this are becoming increasingly important for getting noticed on Facebook.
"Brands and personalities can no longer rely on organic reach alone," he said. "It is generally accepted as best practice to employ a paid, owned and earned model for engagement in social media."
Ogilvy published a paper last week that predicted the organic reach of content published by brands on Facebook will decline to close to zero.
A Conservative Party spokesman said the Facebook campaign was "nothing out of the ordinary" and had been used by politicians in America.
He stressed that the money spent on the campaign was party funds rather than taxpayers' money.