Speaking to Marketing following news that the car manufacturer was "reassessing" its $10m Facebook media spend, Mansell said: "No-one has worked out the value of a Facebook Page and it is up to Facebook and ourselves to work this out."
"But it is short-sighted to say you will pull spend because we can’t prove it. It is astounding that they have decided to make such a fuss before the IPO."
TBG Digital was one of the first three companies to work with Facebook to become an approved API vendor in 2009 with the One Media Manager platform, an automated program for social media advertising.
Mansell said that while it is important to invest in good content on Facebook, paid media helps promote that content and companies should be looking at a combination of the two.
"I think what they (GM) do on their pages is very small compared to other advertisers."
Ford, a rival car manufacturer, has come to Facebook’s defence, stating it is accelerating its efforts on Facebook and other social platforms.
"We've found Facebook ads to be very effective when strategically combined with engagement, great content and innovative ways of storytelling, rather than treating them as a straight media buy," said a spokesman for the car manufacturer.
He added that Ford, which has over 10 million fans globally, has a "strong collaborative relationship" with Facebook.
Ford has the advantage of having a single global brand, while General Motors is a corporate brand with a portfolio of US and European brands.
The GM page has 378,392 fans while its Chevrolet brand has 1,215,042 likes.
According to an industry source who works closely with Facebook, the strategy recommended for automotive clients on the social network is to get loyal car owners to influence purchase intent of their friends, and an effective way to do this is through ads such as Sponsored Stories.