"Parents of young children do often send messages about, 'We can all be friends ... with everybody, ' ... but by adolescence, they start getting more nervous about this and they start thinking, 'Well you should be friends with people like you or like us, ' " said Killen.

CNN: STORY HIGHLIGHTS

Young people, for whom communicating with friends via social-networking sites is often second nature, should be particularly careful about what they post online, Gleeson said.

CNN: Expert: Social networkers risk 'losing control' of privacy

Too often schools fare poorly against these competitors as something that students can hire to be successful and have fun with friends.

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