A Southern Highlands family has been robbed at knifepoint just hours after their daughter posted a picture of her grandmother's life savings on Facebook.

NSW Police have used the armed robbery on Thursday to warn people about the dangers of using social media to post personal information.

Police say the 17-year-old girl was in Sydney helping her elderly grandmother count her savings at about 4pm.

She took a photograph of the pile of cash and posted it on her Facebook page.

But the picture appeared to be the trigger for an armed robbery.

By 11.30pm that night, two men armed with a knife and a wooden club allegedly broke into the teenager's family home in Bundanoon.

Police say the men demanded to speak to the girl about the money but her mother told them her daughter no longer lived at home.

The girl's mother was at home with a young boy and a 58-year-old man.

Police say the men searched the house and made off with a small amount of cash and other property but no one was hurt during the robbery.

The men were wearing dark-coloured clothing, gloves and had their faces covered.

Goulburn police have urged people who use social media to ''take extreme caution when posting photographs and personal information'' on Facebook and other websites.

This comes as the Yellow Social Media Report, published today by Sensis, shows Facebook perched at the top of the tree as Australia's most popular social media website, though NSW users are the least likely to be connected.

NSW residents rank second behind the ACT in using Twitter.

Of social media users, 100 per cent in the ACT use Facebook, compared to 95 per cent in NSW, the lowest in the country.

The report authors surveyed 1015 Australians about their internet and social media usage - of them, 606 used social media.