A big pro of using phone #s is that it allows users to effectively take their social network with them from app to app instead of being locked in. IMO phone nu

A big pro of using phone #s is that it allows users to effectively take their social network with them from app to app instead of being locked in. IMO phone numbers are a pretty terrible system (you would never ask people to remember dozens of IP addresses for all the websites they want to visit but historically people have been expected to remember phone #s for all the people they want to contact), but they are the system we have and it could be worse.