Network-specific messaging tools are another option. I strongly prefer open protocols.
The concept that anyone anywhere can intrude on anyone anywhere else at no financial or reputational cost is ultimately flawed. It works only so long as those with that access are few in number, generally of mutual interest, and act in a largely principled manner.
As numbers increase, and levels of interest and level of principles fall, the system will collapse.
Usenet was the first such network to fall to this dynamic. Email is well on its way. Telephony is into its first years of general intolerability (any direct-dialed universal access, wired or otherwise). Facebook faces this threat less through its lack of filters than the defection of high-affinity users.
Postal mail has its own issues with quality, but the associated costs do in fact impose a minimum bar to malicious content.
