A ruling that Facebook is an advertising medium - and not just a way to communicate - will force companies to vet comments posted by the public to ensure they are not sexist, racist or factually inaccurate.

In a move that could change the nature of the social networking site forever, companies could be fined or publicly shamed for the comments that appear on their Facebook ''brand'' pages.

Last month the advertising industry watchdog issued a judgment in which it said comments made by ''fans'' of a vodka brand's Facebook page were ads and must therefore comply with industry self-regulatory codes, and therefore consumer protection laws.

A media lawyer is warning the Advertising Standards Board's ruling on Smirnoff's Facebook page will put the onus back on companies to be more vigilant about the nature of the comments people are posting to their company pages.

Large advertisers such as Qantas, Telstra and Coles are increasingly reliant on their Facebook pages to get consumers to ''like'' them and get free referrals to their networks of friends.

John Swinson, a partner at law firm King & Wood Mallesons, said the board's ruling ''turned people's opinions into statements of facts''. He said if, for example, a member of the public posted a comment on Smirnoff's site that claimed it was the purest Russian vodka and would lead to success with the opposite sex, and Smirnoff failed to remove it, then the company could be liable on several counts.

''Smirnoff is Australian, not Russian. So, that is false. It may not be the purest, so that could also be misleading. And to imply that you would have greater success with girls would contravene the advertising codes,'' Mr Swinson said.

In a note to clients, Mr Swinson warned that the standards that governed regular TV, radio or billboard advertising might now apply to third-party posts on Facebook pages. And because the competition watchdog was cracking down on claims made by companies in social media - principally by people who are paid to do so - Mr Swinson warned that advertisers could end up in court.

Although the Advertising Standards Board dismissed the original complaint about Smirnoff, which centred on sexism, obscene language and under-age drinking, it ruled industry codes applied not only to what a company posted on its Facebook page but to user-generated comments that followed.