Fooling computer systems usually takes some technical know-how, but using the Internet to fool other people is elementary. No authenticity checks keep me from setting up an email account under any name I choose to use, and then registering for social networking sites such as Facebook. Since I use any name, pictures, and information I choose, I could masquerade as practically anyone, imagined or real.
Adults as well as children need to exercise caution; some people we meet online may be disguising their identities or their true motives. A friend in need might ask to borrow money; an online friend, once she owes you money, might block you from ever seeing her log on again. In-person acquaintance reveals a person's tone of voice, facial expressions, and other nonverbal cues; only a great actor can fake these. Typing a lie is much easier.
There are some valid reasons, such as online dating, to meet and form friendships with people over the Internet. These activities require precautions. I'll emphasize this precaution: Before you entrust an individual with personal information that could be exploited, be sure that you have formed a sufficient friendship with that individual in person.