I think the problem is the cure is much worse than the disease. The cures of using social network tools is often an enclosed, controlled network such as Twitter or Facebook, which puts control of the network in the hands of few.
Instead of trying to replace email, I think we should "fix" it rather than trying to replace it. Some advocate using Web/REST/HTTP to replace email; I think email isn't that doomed.
Remember, the popularity of email is because anybody can start an email service. Nobody can just use Facebook or Twitter; you have to have the approval of an inner circle authority. Imagine if every Web site required MPAA or RIAA control, and they prevented even non-infringing material online using SOPA!