Tell the truth in your online profiles because you cannot communicate effectively if the communication is based on a lie.
Telling the truth in your online profile makes the experience better because your friends have an accurate profiles of who you are.
The purpose of online social networks is communicate with friends.
Lying in your online profile prevents effective communication because the people you are communicating with are basing their communication on lies.
Since lying in your online profiles makes the communication ineffective it is bad netiquette to lie in your online profile because it detracts from the experience.