Thousands of Australian teenagers are unwittingly revealing their mobile and home telephone numbers online via Facebook when they thought the numbers will not be seen anywhere else.

The phone numbers of Facebook users are being disclosed to others when users respond to event invitations which are not made private.

There have been numerous cases when Facebook users who either lost previously saved numbers or got a new phone asked for their friends' numbers on an event invite's wall. Once users respond, the numbers are disclosed to people outside private networks.

Facebook event invites are typically used for inviting friends to parties, but many users have found they can also use them to collect telephone numbers of friends. However, some users fail to secure the wall for friends-only access, Fairfax Media reported.

In about five minutes, Fairfax found thousands of numbers for teens in New South Wales, Victoria, Western Australia, Queensland and South Australia just by searching public event invitations for words such as "mobile."

Cyber security expert Susan McLean told Fairfax said Facebook users across all ages and were disclosing their numbers without even realising it.

"It never ceases to amaze me that people don't tend to think," she said. "Most of these people wouldn't give their mobile number to a person in the real world."

Fairfax advised Facebook users to send phone numbers via private messages instead of posting to any wall.

Better yet, Ms. McLean notes, avoid using Facebook to post your phone numbers to the social networks as others could gain access to it.