With barely hours to go before the much-hyped flotation of Facebook shares on the US stock market, yet another leading company has announced plans to stop advertising on the platform, reigniting the debate about what the future holds for the social network as an ongoing business concern.

US car manufacturing giant General Motors is the latest high-profile advertiser to call time on its Facebook advertising on the grounds that the company could not find any evidence that it translated into an increase in sales in the showrooms.

Reports suggest that GM paid around $10 million to advertise on the social network last year. However, this paled into insignificance compared with its overall advertising budget, which is believed to be in the region of $1.3 billion a year.

Facebook's Mark Zuckerberg

And the company isn’t the first to voice its reservations about Facebook’s viability as an advertising medium. Only earlier this month, Korean automotive rival Kia Motors expressed similar reservations about the effectiveness of Facebook advertising.

Despite throwing in the towel on its Facebook ads, General Motors has nevertheless decided to maintain a presence on the site by continuing to use its branded Facebook page, which it still sees as a valuable online public relations tool.

But what significance, if any, does this recent series of widely publicised Facebook advertising failings mean for PR?

It has long been recognised that social networking sites have been much closer aligned to the marketing objectives of PR than those of advertising. So on the face of it you’d have to say that these recent cases would have very few implications for public relations.

What’s more, advertising success on social media very much depends on who is doing it and how they go about it – and it may well be that Facebook is highly effective for some advertisers, but not so useful to others.

But what is concerning from a PR perspective is the undermining impact that examples such as General Motors and Kia could have on other advertisers toying with the idea of advertising on Facebook.

Not only could this have a knock-on effect on confidence amongst their public relations counterparts, but also on the prospects for Facebook as a sustainable long-term business model.

Just as MySpace and Friends Reunited fell out of favour almost practically overnight, it is therefore not inconceivable that Facebook could go the same way at some unknown time in the future.

And for many public relations agencies this will be a case of back to the PR drawing board and a new online marketing strategy for the next big thing.