Of 14 websites tested on behalf of the European Commission, just two -- Bebo and MySpace -- ensure the required controls to make sure "potential strangers" cannot gain access to profiles.
"I am disappointed that most social networking sites are failing to ensure that minors' profiles are accessible only to their approved contacts by default," said European Commission vice president Neelie Kroes in a statement.
The authorities in Brussels say the number of minors who use the Internet and subscribe to social networking sites is growing -- currently 77 percent of children aged 13-16 and 38 percent of those aged 9-12.
Kroes said she will urge online social networking site owners to make the changes in a "self-regulatory framework" under discussion.
"This is not only to protect minors from unwanted contacts but also to protect their online reputation. Youngsters do not fully understand the consequences of disclosing too much of their personal lives online," she said.