(and I wouldn't say the social networks where profiles don't need to be connected to real names are necessarily any better)
Sure people can change their nicknames, but people change their real names too. I've gone by three different 'real' names throughout my life. That may not be super common, but people getting married and changing their name at least once is certainly not rare. The way I see it, a 'real' name is only more real than the others insofar as it's the name the government uses for you. But that sort of realness isn't relevant for social purposes. For social purposes, the 'realest' name is the name people call you.
Sure, it's possible to stay in touch with a long list of monikers and sometimes even not much more difficult, but (going back to the OP I responded to) it's possible and often no more difficult to ruin people's lives across pseudonymous services too. Lack of real name is probably more of an impediment to the casually interested old friend than the concerted hate campaign.
I'm not sure a teenager has the same value. Anyone under 18 shouldn't have real identifying names them.
When facebook came out you had people isolated into networks of schools. Those structures provided better protections and freedom. The transition to fully public with forced real names made facebook into something not for kids but great for older folks.