As far as Plato and Aristotle are concerned, I'm not sure. But by the time Augustine was writing, the educational systems that grew out of schools founded by Plato, Aristotle, and their successors had spread around the Mediterranean, and graduates from these institutions were very much in contact with each other.

Education at the highest level (called, in Greek, paideia) taught students philosophy, but it also helped them make contacts and form professional networks that were crucial to success in public life in the ancient world. These networks were maintained through constant letter writing, and we know a lot about them because hundreds of these letters survive.

These networks helped with political connections and favors, but they were also crucial to spreading philosophical ideas. Copying a book by hand could take months (or years), so authors didn't make hundreds of copies of their books to sit in stores on the hope that someone would buy them. Instead, they relied on their networks from school to get the word out that they'd written something important, and those of their friends who were interested would send an educated slave to visit for a few months (or years) in order to copy the book. Once a few people had copied your book, you could hope that their friends would send slaves to copy it from them, and it would start to spread to a wider and wider audience that would eventually reach beyond your personal network of school friends to a wider audience.

As a result, some books spread very quickly - the Life of St Anthony only took a few decades to spread around the entire Mediterranean and be translated into Latin (from Greek). This was only possible because of the networks of friends and contacts, maintained through letter writing, that tied the educated elite around the Mediterranean together into a loosely structured community.

It's not terribly different from social networking today, really - just a little more lag between message and response.

For more, see Grafton and Williams, Christianity and the transformation of the book, and M. Williams, the monk and the book. On late Roman education and networks of letter writing more generally, E. Watts, City and school and P. Brown, Power and Persuasion.