As others have pointed out, though, an open social protocol would quickly degenerate into a wretched hive of ads and spam. The underlying problem to solve is identity, which the existing social networks attempt to solve through real-name policies, social conventions, and pro-active policing, to various degrees of success.
This is going to be unpopular to say on HN, but there would be upsides to a government-issued internet ID card, that allowed to someone to declare unambiguously which actual human they are (ideally on an opt-in basis).
Would the inevitable abuses of this power be worth the benefits? Probably not. But authenticated identity is the core problem that needs to be solved for an open social protocol to really work; without it, you're trying to craft TCP/IP without IP addresses.