First off, this may not be brief. Deal with it.
Many many years ago, I had a 1200 bps modem. Seems absurd to me, considering I've been a part of a company with a 1,073,741,824 bps connection, which speaks somewhat effectively to the rate of advance of communication speed over my time following such things.
Another note... This post comes with a geek content advisory warning. You may or may not find it extreme, just be advised that I may drip nerd all over your brain.
So on to the main point... many-to-many communication networks. Right now we are calling them "social networks." I've been doing work consulting with people about how these contemporary tools can help them do business, which has gotten me thinking.
Whatever you call them now, for me "social networking" all starts with that 1200 baud modem. Of course, the history of humanity and society is based on people building social networks, but my own personal history starts at the beginning of digital communication, so it's a convienent place to start.
You see, back before 'the world wide web,' and even really before AOL took hold, there were BBS's. For those who don't know, bulletin boards were mostly enterprising people running their own home servers with say, 4 to 20 phone lines (not cheap for a non-commercial techie hobbyist) where other locals with home modems would dial in and chat and or waste time on door games or whatnot.
When I was 11, the BBS's were all-fascinating. I don't even know how I FOUND the numbers, but there were 30 or 40 around Memphis, which is not the largest nor most fast-adopting of cities. It was amazing to be able to use your home computer to chat with NINETEEN other people online at the same time. My sister and I damn near got into fistfights for modem time, her to check email from boyfriends and I to get on the BBS's. The many-to-many communication is addicting, and the early 90's was when we really noticed the possibilities.
It should also be noted that, like usenets and early AOL, private BBS's were innundated with porn. It attracted people to pay subscription fees, which is how individuals could afford bulk phone lines. Goes to show, there was always digital porn, and always will be, I suspect. Admittedly, with a 1200 baud card you had to wait 5 minutes for a 120k image to download...
And then BAM! a paradigm shift happened.

That was AOL, and the World Wide Web. Why not take all of those private local servers and phone logins, and link them to that weird cross country information backbone that had let research universities talk for 20 years. Now you can have chat rooms of FORTY EIGHT people, and you can chat with someone in another state, even across the country!
The real evolution here was inherent in the scale of the communication. On a BBS, we were stuck with the same small population of locals that knew how to find the board. The tiny subculture was inherently just communication within a set group. But the Web? A million people... you could chat with someone new every hour and never talk to the same enduser twice.
And this led to segregation, to segmentation, to users making social choices and developing online communities. Usenets. Private topic based websites, multiplayer online games (yep, oldschool Neverwinter Nights...) . Yeah, crude compared to today, but ASTOUNDING in 1994 (and a brief fact from the wiki... when NWN launched in 1991, people payed $6 PER HOUR to play)
Now, I never was an extensive user of AOL. The whole "walled online community" gave it the benefits that led to it being the earliest big peer-to-peer social network, but I personally couldn't stand the highly invasive client software and the barely permeable proxy between the public internet and "AOL space"
You can point to any number of intermediary technologies that were the next evolution in digital communication, but really the thing that got me originally thinking about this whole subject was actually AOL Instant Messenger. Yeah, I was on IRC, but IM was the popular service that all of the non-geeks used too. IM had such overpowering market penetration, at least amongst a certain age demographic, that suddenly an entire generation grew accustomed to being chronically digitally in touch.
IM got me thinking for just that reason. It wasn't, unlike the rest of the things I've been discussing, a many-to-many network, it was more one-to-many. It didn't have the inherent social subdivisions that were fast becoming part of the internet. But it fundamentally changed how we communicated and kept up with our immediate and even extended social circles. You could communicate information instantaneously to any number of people, and may or may not need a response. It taught us all how to text.
And more pertinent to my point, it was a nearly universally shared experience in people's use of communication, which has nearly vanished now (of course my AOL IM account talks to my google chat now, so it's just still evolving, not dead.)
A Brief footnote on another vital piece of the social network family tree from the late 90's. Open Diary. The year was 1998, and everyone I knew was a geek who wanted to be a writer. And then there was this website, that was ALL geeks who wanted to be writers. No one knew what a "blog" was back then, or that emo high-school geeks writing about their depression would cause another evolution in communication.
I'm exaggerating a bit the impact that Open Diary had, but it was definitely a public social site, and one of the first public blog sites. And of course I mention it here because yes, I was one of those geeks who likes to write.
Well, that essentially gets us up to the landslide of many-to-many communication tools we have today. The framework that we saw in the 90's BBS's (mail, chat, games, picture sharing, usenet) has essentially continued completely unaltered except by the addition of personal messaging and blogs/bio pages. Facebook, with 25% of all internet traffic, has vastly more advanced technology than a dial-in server, but people use it in a VERY similar way.
So, the question is, where will the next 20 years take communication?