I don't see what real life names have to do with keeping in touch. I've kept in touch with people for 15+ years using IRC nicknames. In a few cases I don't know the other person's real name at all.

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.

I mean "here's all the people you went to school with" was literally Facebook's raison d'etre. If I was relying on stored phone numbers or email addresses I'd be a lot less likely to be in touch with some of them (including those whose numbers I still have!)

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.

It's a valid point and I have found value in looking up past friends on facebook.

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.