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.