Chat programs and talkers and local messaging go back to the 1960s. We had floppy disks in 1970 (the VCR wasn't until 1971 and the Walkman was 1979) - and the @ was chosen for addresses by Ray Tomlinson writing email software in 1971. What Plato did have - which is probably what made an impact on Ray Ozzie that we can see in Lotus Notes and Groove - was the original Notes, which started as a bug reporting system and turned into a conferencing system, along with Talkomatic, which was different because it didn't wait for a whole line of text before it sent your message, and an online community built around them. I saw another descendant - DEC Notes - in the late 1980s, so discovering Usenet felt like coming home.
What's really remarkable is how new we think all this is. The PC world's failure to learn the lessons of the mainframe and workstation world about basic concepts like multi-threading, multiple user accounts, security and communications - let alone the more sophisticated tools that were developed long before desktop computers had to earn their living - have meant that our industry repeatedly re-invents the wheel without seeming to have any idea that there was a wheel before. At least Linux reprises an OS first coded up in 1969. It might have been clunky but I was doing mobile email in 1993 with an HP OmniBook 300, a Motorola CELLect mobile phone and CiX. What other industries routinely ignore the knowledge gained in previous developments? Never mind code re-use, we don't seem to have much interest in re-using successful designs and building on them. The ground behind most development teams is streaming with bathwater and covered with babies.