Except for Facebook and LinkedIn, which require real names, many of the kids I know through my son have alter egos they use for posting on various social media and forums. They are generally careful not to provide more than time zone info about the physical location of that alter ego. My son has done searches to make sure he couldn’t connect his alter ego to his realname. Some of them have multiple alter egos, but my son and daughter each have one that they use frequently. Their friends do typically know their alter ego names. My daughter has a couple extra, because she publishes fan fiction and tweets about fan fiction under those names.
Some of the kids who are applying to colleges have to think about whether to breach the wall between themselves and their alter ego when applying. For example, my son enters 3D modeling competitions under his alter ego, and will probably want to submit some of that work to some colleges. Some computer science related stuff also, though he has separate GitHub accounts for his alter ego and his realname, and the more polished stuff goes under his realname. He says he’s had discussions with some Class of 2014 friends about how they “merged” their personas after high school.