Social networking giant Facebook looks set to let children under 13 use its site legally, which could attract millions of new users.

The move has been praised by a social media expert who said it would provide new opportunities for businesses looking to engage with a younger audience.

"From that point of view that's good for us and good for our clients," Eight80 Social Media director Adam Smith said.

However, he said it was likely Facebook would put restrictions on what advertising younger users were exposed to. A senior Facebook employee in Britain admitted many under-13s were already using the site.

Head of policy Simon Milner told the Sunday Times there was "reputable evidence" of children under 13 lying about their age to get on to Facebook, some with their parents' permission or help.

"We have a strict under-13 rule because of legal issues in America, and we apply the same rule all over the world. But a lot of parents are happy their kids are on it," the Facebook employee said.

Last year's annual Auckland University schools census, which collects data from more than 20,000 students, showed about 60 per cent of 12-year-olds, 40 per cent of 11-year-olds and about a quarter of 10-year-olds had a Facebook page.

There are more than two million Facebook users in New Zealand.

Facebook chief executive Mark Zuckerberg said last year children under 13 should be on Facebook. "It will be a fight we take on at some point. My philosophy is that for education you need to start at a really, really young age."

NetSafe executive director Martin Cocker said Facebook had flagged its interest in operating in the younger market at a conference he attended last year.

"[Facebook will] have to establish processes ... appropriate for younger children. At the moment they can get away with saying, `we operate in a certain way because we cater for people 13 years and up', but, of course, we know there are many children using the site that are younger than that."

- © Fairfax NZ News